The Government School of Mines,
Jermyn Street, September 14, 1857
My dear Tyndall,
In the following pages I have give you some account of the experiments and observations upon the structure of glacial ice, which, at your suggestion, I commenced during our sojourn at the Montanvert this autumn. No one knows better than yourself how much these subjects grow under the hands of the inquirer, and how little claim my brief investigations have to the character of completeness. Nevertheless my conclusions, so far as they go, are based on such clear and decisive evidence, and are so totally opposed to the views entertained by the highest authorities, that I feel I shall be doing more good by publishing than by withholding them.
I will in the first place state what I have myself observed with regard to the structure and the permeability of glacier ice, and afterwards I will compare my results with those which have been arrived at by others.
Structure of Glacial Ice.A mass of ice freshly extracted from any part of the Mer de Grace, the Glacier du Géant, or the glacier of La Brenva, at a depth of 8 or 10 inches from the surface, always presented the following characters when examined either with the naked eye, or with a lens of a magnifying power of thirty or forty diameters.
It broke with a vitreous fracture; and when the surface was made even, either with a sharp knife, or by rubbing on a warm surface, it  appeared perfectly smooth and glassy, exhibiting not the least trace of fissures. Minute shallow pits, however, were scattered over it, and became particularly obvious when a coloured fluid was poured on to the surface and then wiped away again, inasmuch as under these circumstances every pit retained a very small portion of the colour.
The mass was, as usual, traversed by a larger or smaller number of parallel blue veins (whose lenticular form was almost always very apparent, particularly in the Brenva); and when a thin section was made perpendicular to the plane of the veins and viewed by transmitted light, it became obvious that the ice formed one continuous mass, without fissures or interruptions of continuity of any kind. It contained, however, a multitude of small, closed, and perfectly distinct chambers, and it was to the absence or rarity of these in the course of the veins that the latter owed their transparency and blueness.
The form and contents of these chambers were exceedingly remarkable. In a blue vein, and in those parts of the intermediate "white ice" which were contiguous to a blue vein, they were always round or oval disks, with extremely flat and closely approximated sides; so that, viewed in one plane, they looked like circles; but in a plane at right angles to this, like narrow parallelograms. In the white ice midway between the blue veins, on the other hand, I very generally noticed an irregularity of form, which was in many instances so great that the cavities appeared to be ramified. The walls of the chambers very frequently appeared to be a little roughened, or, as it were, frosted.
Every chamber, without exception, which I carefully examined contained both water and air. The former was commonly present in larger quantities than the latter, which swam as a bubble in the water, and could very often be made to move about in the chamber like the bubble of a spirit level. It seemed to me, though I will not pretend to lay it down as a rule, that the air was more abundant in proportion to the water in the more irregular chambers. Where the air was in large proportion to the water, the bubble of course became more or less completely supported by the walls of the containing cavity, and to a certain extent assumed its form; but where, as in the majority of cases, the air-bubble was small in proportion to the water, its figure was spheroidal, and totally different from that of the containing cavity. I mention this particularly, because, as I shall show below, the chambers (which for distinction's sake I will term the "water chambers") have been confounded with the air-bubbles, and the form  which is characteristic of the one has been erroneously ascribed to the other.
I had no means of measuring the dimensions of the water-chambers, but at a guess I should say they varied from a tenth to a fiftieth or a sixtieth of an inch in diameter.
The line of contact of the water in the water-chambers with the ice was optically perfectly well defined, and easily distinguishable. Hence I have no hesitation in saying, that if canals or fissures of any appreciable size filled with water had existed in the ice, I must, with the magnifying power employed, have discovered some trace of them; but I repeat, nothing of the kind was discernible in perfectly fresh ice.
If the existence of fluid water dispersed through its substance in closed chambers is shown by future observations to be a universal character of glacier ice (and I cannot imagine that a structure universally prevalent in the Mer de Glace, the Géant, the Brenva and, as I shall show by-and-bye, from M. Agassiz's figures, in the Aar glacier also, is a mere local peculiarity), it appears to me to be a fact of primary importance. For what I have described is the structure of the unchanged ice of the glacierof ice which has been protected from the solar or atmospheric influences by that which covered it; and it must be remembered that the ice which is within a foot of the surface on the Mer de Glace opposite the Montanvert must have formed a part of the very depths of the tributary glaciers. In other words, the ice which is at this moment, say a hundred feet below the surface in the Glacier du Géant, will, in consequence of ablation, form the superficial ice of some part of the Mer de Glace years hence. Consequently, unless it can be shown that the substance of a glacier, as it approaches the surface, is exposed to some influences capable of developing the water-chambers and their contents, it is to be presumed that the structure found near the surface in the lower part of a glacier is the structure which prevails throughout the thickness of the higher part; and hence that the structure described is that of unaltered glacier ice in general. This conclusion, as I shall immediately show, is directly confirmed by the boring experiments and by the figures of M. Agassiz.
M. Agassiz's deductions, however, are totally at variance with mine; and he is so generally quoted as an authority in these matters, that I feel compelled, however unwillingly, to enter into a detailed criticism of his views, which are contained in the following extracts from his 'Systeme Glaciaire,' numbered, for the sake of more convenient reference, in successive order.
 (1) "At its origin, near the Névé, the compact (or proper glacier ice) contains, like the ice of the Névé, a notable quantity of air. But there is this difference between the two, that in the compact ice the air, instead of being distributed through the whole mass, is united in small perfectly circumscribed bubbles, whilst the interspaces of these bubbles are perfectly transparent, so that without being as diaphanous as ordinary water-ice, the compact ice has not the opacity of Névé ice. Moreover, it is more compact, and what is especially characteristic, it presents no trace of granular structure: a fragment exposed to the action of heat does not become resolved into grains of Névé, but breaks up into angular fragments.
"This difference of structure is accompanied by a greater impermeability; water no longer traverses the mass with the same ease and uniformity, but is seen to follow in preference certain angular routes which are the capillary fissures."P. 151.
(2) "The means employed by nature to maintain this amount of plasticity and compressibility in glacier ice is the water which circulates throughout the mass, and which, while it lubricates it, contributes to maintain within it a constant temperature during the greater part of the year."Pp. 152, 153.
(3) "Superficial fissures which must not be confounded with the capillary fissures.
"When during a fine summer day one travels over the upper regions of the compact ice (about the region of the Abschwung, on the Aar glacier), a continual crepitation is heard on all sides. It is caused by the bubbles of air which on approaching the surface escape through the ice, where they have been dilated by the effect of diathermanicity, and cause the parietes of the ice to burst when they are no longer sufficiently strong to resist the dilatation of the air."P. 153.
(4) "The air-bubbles undergo no less curious modifications. In the neighbourhood of the Névé, where they are most numerous, those which one sees at the surface are all spherical or ovoid; but by degrees they begin to be flattened, and near the end of the glacier there are some which are so flat, that they might be taken for fissures when seen in profile. The drawing, pl. 6, fig. 10, represents a bit of ice detached from the gallery of infiltration. All the bubbles are greatly flattened. But what is most extraordinary is, that, far from being uniform, the flattening is different in each fragment, so that the bubbles, according to the face which they offer, appear either very broad or very thin. I know of no more significant fact than this, since it demonstrates that each fragment of ice is capable of undergoing  in the interior of the glacier a proper displacement independently of the movement of the whole."P. 167.
(5) "The same flattening of the bubbles is found at a greater depth. While engaged in my boring experiments, I observed attentively the fragments of ice brought up to the surface by the borer. I found in them almost flat bubbles, perfectly similar to those of the fragment figured above, at all depths from 10 to 65 metres. I observed, besides, that in the fragments which proceeded from a great depth, all the bubbles without exception were strongly flattened, whilst at less depths there were some less compressed and even altogether round, as at the surface.
"It follows, hence, that a strong pressure is exercised in the interior of the glacier."P. 167.
(6) "I ought also to mention a singular property of these air-bubbles, which at first was very surprising, but afterwards admitted of very satisfactory explanation. When a fragment containing air-bubbles is exposed to the action of the sun, the bubbles insensibly enlarge. Soon, in proportion as they enlarge, a transparent drop shows itself on some point of the bubble. This drop in enlarging contributes its share to the enlargement of the cavity, and as it progresses it predominates over the air-bubble. The latter then swims in the midst of a zone of water, and incessantly tends to reach the most elevated point, at least if the flatness of the cavity does not hinder it."Pp. 167, 168.
(7) In a note appended to this passage, M. Agassiz speaks of the irregularity of the walls of some of the bubbles, and adds, "The same effect has been produced upon the bubbles of the fragment fig. 10. There also all the bubbles have enlarged by diathermanicity, and a little drop has developed in the middle of each. But as the cavities are very small, the drops do not yet move freely in their cavity."
It will be observed that in Nos. 1, 4, 5, 6, M. Agassiz confounds together the water-chambers and the air-bubbles under the common term of "bubbles," and he affirms (6) that the presence of water in the "air-bubbles" is the effect of exposure to the sun's rays, and of the different diathermanicity of air and ice.1 A careful analysis of M. Agassiz's facts, however, is very instructive. In the first place, I recognize in his fig. l0, pl. 6, a fair, though rough and sketchy, representation of the general arrangement and form of the water-chambers with their contained air-bubbles The chambers are as usual flattened, but the artist has rightly represented their contained air-bubbles as  spheroidal. The strangest thing is, however, that M. Agassiz has taken the air-bubbles for drops of water, and the drops of water for air-bubbles, as any one who is familiar with the microscopic appearance of bubbles of air will see, on comparing the description in (7) with the figure l0. In the next place, I repeatedly exposed thin plates of ice to the sun, carefully watching the air-bubbles, without being able to observe the phenomena detailed by M. Agassiz in (6); and I must frankly confess I do not understand how such changes as those described are reconcileable with the commonest properties of ice and air. How do the bubbles enlarge when exposed to the sun? M. Agassiz has already admitted that the chambers are closed (1), and we know that ice is not readibly distensible; and therefore I hold it to be impossible that the bubbles should visibly dilate before the melting of the adjacent ice; and as to enlarging by the melting of the ice-wall, the fractional difference between the volume of water and the ice from which it proceeds, would be wholly imperceptible on such a scale. With regard to the explanation of the crackling noise given in (3), I can only say that I have repeatedly watched a thin lamina of ice melting, both by transmitted and reflected light, and that I have seen the walls of the chambers reduced to the thinnest pellicle without being broken by pressure from within. The air-bubbles escape quite quietly as soon as their wall is perforated. Furthermore, the cavities left where the air-bubbles have been, are not fissures at all, but, as I have said above, rounded pits. Indeed, this is a necessary consequence from M. Agassiz's own statements with regard to the shape of the bubbles.
M. Agassiz affirms in (5), that ice brought up from a depth of 65 metres was perfectly similar in structure to that represented in his figure 10. The fact is important; but surely it alone affords sufficient evidence that "diathermanicity" has nothing to do with the formation of the cavities and their watery contents. And indeed in (4) this same piece of ice (fig. 10) is said to have come from the "gallery of infiltration," a cavity perfectly shaded, and bored many feet below the surface of the glacier. So that either this figure does not represent the structure of the glacier at this point, or the structure is unaltered, and diathermanicity has nothing to do with it.
It follows, therefore, that there is no evidence to show that the influence of solar radiation has anything to do with the structure; on the contrary, M. Agassiz's facts strengthen my case.
If it be the universal character of glacier ice to be full of closed cavities containing fluid water and air, it becomes a matter of extreme interest to ascertain how the air and the water come there; how it is  that the water retains its fluidity, and how it is that the water-chambers are compressed. It may seem a common-place comparison, but the ice and its cavities containing water remind me of nothing so much as a Gruyère cheese, in which one so often meets with closed cavities containing fluid and air. Let the Névé represent moist curds and the glacier valley the cheese-press, and the analogy is perhaps closer than it looks. But these are questions for you to solve; and I will only venture on one other supposition, viz. that the water-chambers have the value of a register thermometer, indicating that the minimum temperature to which the mass of a glacier descends is never for long less than 32°; otherwise I cannot conceive how the water should remain fluid; and if it were once frozen, how could it melt again?
M. Agassiz makes a very important observation in (4), and one which I am glad to be able to confirm in the main. I took some pains to ascertain the general direction of the planes of the water-chambers, and I found that in the substance of the blue veins they were sometimes parallel to the plane of the latter, while in the white ice their planes were always more or less inclined to the veins, usually forming an acute angle, and never, so far as I have seen, a right angle with them. Furthermore, as Prof. Agassiz points out, the water-chambers are arranged in groups, all the members of the same group having parallel planes, while their direction is more or less inclined to that of neighbouring groups. It seems to me very probable that, as Prof. Agassiz suggests, the different directions of the planes of the cavities may indicate internal changes of place of segments of ice corresponding with the groups; but, as I have already said, no fissures separating these segments are to be found in the deep ice of a glacier, and hence we cannot with propriety speak of them as "fragments."
Such is the structure which I have found to obtain in all "deep" glacier ice, by which I mean, all ice situated more than a few inches below the surface. It is as solid as glass or marble, and as devoid of any but accidental and gross fissures. The glacier, however, where exposed to the atmosphere, presents what may be called a "superficial layer" of very different character. Every one who has had occasion to cut an escalier, must have been struck with the difference between the resistance to the ice-axe at the first blow and that at the fourth or fifth. At the first, the jar to the hand is slight, and fragments of ice fly in all directions; but at the last, one might almost as well be hewing some hard though splintery wood. The reason of this at once becomes apparent on examining the superficial ice. It is composed of larger or smaller granules of exceedingly irregular form,2 separated by very obvious fissures, but nevertheless so fitted into one another as to cohere with some firmness. The distance to which the fissures extend into the interior of the glacier (and hence the thickness of the superficial layer) varies a good deal; 7 or 8 inches is perhaps rather above than below their average depth; but however this may be, the important fact is, that whenever you clear away the superficial layer, you find beneath it what I have termed "deep" icethat is, ice in which neither fissures nor granules are discernible; ice which tends to split parallel to the veins, and shows no disposition to break up into the angular fragments so characteristic of the superficial layer.
It has been said that mere optical examination is insufficient to disprove the existence of fissures in the deep ice, and that such fissures are present, though invisible in consequence of being filled with water. I have already shown that the line of contact of water and ice is optically well marked, and that there is every reason to believe that even the finest fissures would be visible under a sufficient magnifying power; but those who maintain the porosity of glacier ice, rest chiefly on the results of experiments made with coloured fluids. It is said that glacier ice becomes infiltrated throughout its substance with extreme readiness, the coloured liquid traversing fissures which are more particularly developed in the course of the blue veins. It became necessary, therefore, to repeat these infiltration experiments; and for this purpose, as you will recollect, I made use of the logwood infusion which you had prepared, and which by its combined clearness and intensity of colour was excellently fitted for the object in view.
If a little of the infusion were poured upon the natural surface of the glacier, it immediately soaked in, spreading itself in all directions between the granules (but more rapidly, as I often observed, in directions parallel with the veins), and staining the whole thickness of the superficial layer. Whatever quantity might be poured on to the surface, however, it penetrated no further than the superficial layer (unless there were some obvious crack in the deeper ice); and when the latter was cleared away with the axe, and the surface of the deep ice washed or even carefully rubbed with the hand, not a trace of the infusion could be found in it.
If a piece of the deep ice containing several blue veins were allowed to soak in the logwood infusion until it nearly melted away, it remained unstained, and either wiping it or passing it quickly through clean water rendered it perfectly clear and stainless.
 But it is said that if cavities be made in the glacier and filled with a coloured infusion, the latter will soon, by means of the capillary fissures, infiltrate the surrounding mass. To determine this point, I selected a spot upon the north wall of a crevasse, just opposite the Montanvert, and between the centre and the west shore of the Mer de Glace, where the veins were well developed, their planes having a general north and south direction, but dipping at an angle of about 70° towards the centre of the glacier. On the northern aspect of the ice I cut away the superficial layer, so as to form two faces of a cube of about a foot in the side on the deep ice. One of these faces looked westward, and was consequently nearly parallel to the cleavage; the other looked northward, and was therefore nearly perpendicular to it. Perpendicular to the west face, and therefore to the structural planes, I bored a hole with an auger, about an inch in diameter and 9 inches long, and just sufficiently inclined to the horizon to hold the infusion of logwood, with which I filled it. I then thinned away the north. face of the cube very carefully until the north wall of the hole was less than 2 inches in thicknessuntil, in fact, I could see the dark fluid through the substance of the several blue veins which it traversed with perfect distinctness.
For two hours not the slightest trace of leakage or infiltration into the substance of the ice forming the walls of this cavity could be observed; and the contour of the contained liquid remained perfectly sharp and well defined. It then began to leak at one point near its upper end through a small crack in the white ice, which led directly outwards. The liquid spread neither up nor down in the crack Four hours afterwards no change whatever had taken place in the liquid contained in the lower part of the hole. At this time you joined me upon the ice, and you will recollect that I carefully thinned away the wall with a sharp knife until in some parts it was not more than l/4 of an inch thick. Still no infiltration occurred. The knife at length accidentally penetrated the wall, and the liquid at once flowed out. I then poured some clean water through the hole, and all trace of the coloured infusion was at once so thoroughly removed, that, on cutting away one wall, the other appeared perfectly clean and of its natural aspect.
I have given the details of this one experiment in order to show in what manner all were made; but it is unnecessary to be equally prolix with regard to the others. Suffice it to say, that, whether the holes were bored perpendicular to the structure or parallel with it, or at any intermediate angle, whether in white ice or in a blue vein, the result was precisely the same, not a particle of fluid making its way into the surrounding substance of the ice along the veins, nor in most  cases in any other way. Occasionally a leakage would take place in the manner described above, but the fissures in these cases were gross and visible, and their direction had no reference whatsoever to that of the structure. Indeed, as the leakage always took place towards the surface, and not into the depth of the ice, I am inclined to think that these cracks were produced in cutting the ice to thin away the outer wall of the cavity.
I repeated these experiments in the neighbourhood of the Grand Moulin; on the Moraine du Noire, somewhat higher than the Couvercle; and on different parts of the Glacier du Géant, and everywhere with similar results. Furthermore, having carefully bored a vertical hole in the deep ice of the Mer de Glace, opposite the Montanvert, I filled it with the infusion, and having covered over the aperture with a roof of ice-blocks, I left it until the next morning. It rained hard during the night; and on revisiting the spot after an interval of about fifteen hours, I found that the covering blocks had slipped off, and that the liquid occupied only about the lower two-thirds of the cavity. No trace of infiltration could be discovered; but the lower part of the cavity had changed its figure from cylindrical to irregular and botryoidal. 1 conceive that the sinking of the fluid must be accounted for by the enlargement of the cavity consequent upon this botryoidal excavation of its walls; and I suppose that the ice-blocks proving an insufficient shelter, the rain poured into the hole, and keeping up a constant supply of comparatively warm liquid, eroded its walls in the way described. However this may be, the fact that the liquid had produced a fresh surface for itself, is important, as it shows that the absence of infiltration through the veins intersected by the cavities containing the coloured infusion is not dependent on a condensation of their walls by the auger.
To eliminate any error of this kind, however, I took a small block of the deep ice, and with a sharp knife fashioned it into a cup, whose walls varied in thickness from 1/4 to 2/3rds of an inch. Filled with the infusion and surrounded with ice, this cup remained for two hours without showing a trace of infiltration along its structural planes.
I can only conclude from these experiments, that the chief substance of a glacier is as essentially impermeable as a mass of marble or slate; and that though it may be traversed here and there by fissures and cracks, these no more justify us in speaking of glacier ice as "porous," than the joints and fissures in a slate quarry give us a right to term slate porous. We do not call iron porous because water runs out of a cracked kettle.
The extreme porosity of what I have termed the "superficial  layer," however, is no less certain, and inasmuch as this layer is continually and rapidly wasting away at its surface, it must be as constantly reformed from the solid glacier ice beneath.
The fact observed by Prof. Agassiz, that under a moraine (that is, where covered and protected by stone and gravel) the superficial ice is of the same character as the deep, suggested the idea that the superficial layer is the result of the operation of atmospheric influences; and that just as a bed of impervious rock becomes broken up into fragments, separated by permeable interstices, down to a certain depth wherever it is exposed to the atmosphere, so the glacier ice when left unprotected undergoes a similar weathering and disintegration. 1 submitted this notion to the test of experiment in the following way:Not far from the upper end of the Moraine du Noire, and on one bank of a stream which cuts its way down the Glacier du Géant, I cleared away the superficial layer and cut out a block of the deeper ice, which was then divided into two equal portions of irregular cuboidal form, and about 8 inches in the side.
The logwood infusion was poured on both of these, and was retained only by such portions of the superficial layer as had been allowed to remain. Water poured on to the blocks ran off them as it would run from marble or glass, sinking only into the remains of the superficial layer. I then placed the two blocks side by side, on an elevated ridge of the glacier, with their natural upper faces turned towards the sun, at this time (1.15 P.M.) shining brightly; the one block I left without protection, while the other was just covered by a stone of 4 or 5 inches in thickness, resting upon its upper face. At 1.40, that is to say in less than half an hour, I removed the block of stone and poured the infusion over both pieces of ice. The covered one could be as little infiltrated as before, while the face of the uncovered became at once beautifully injected, the fluid instantly running into a network of little superficial fissures which had developed themselves, and out of which the infusion could be only partially extracted by washing
Both pieces of ice were well washed, and the stone was replaced on the one, while the other was left uncovered as before.
In the course of the ensuing half-hour I examined both blocks several times. The covered ice remained unchanged; but in the uncovered, the fissures extended further and further into the mass, which gradually assumed throughout the granular aspect of the superficial layer. Water poured on its surface soaked into it immediately, and a small quantity of the infusion spread out, the moment it reached the block in the most beautifully ramified figure through the fissures. Particularly large and apparent fissures could thus be  frequently observed traversing the middle of the blue veins. At length the fissures extended completely through the mass, which thus became truly sponge-like. Water poured on its surface, filling the interstices, gave the mass a clear and semitransparent aspect, though by no means to be compared to that of a blue vein. But as soon as the supply of water ceased, the fissures of the side uppermost immediately began to lose their water, which drained away below, and becoming filled with air, a whitish opaque hue succeeded. On reversing the block suddenly, what had been its under surface appeared at first clear, but the water soon deserting it, it rapidly whitened, while the previous upper and white surface became clear. Water poured upon the upper surface, traversed the mass and flowed out again below with the utmost ease. In fact it is impossible to conceive any more striking contrast in these respects, than that between the freshly extracted ice-block (or that which had remained under cover) and that which had been exposed.
So far as it may be permissible to draw a conclusion from the few experiments I made, I should say that direct exposure to the sun has much influence on the rapidity of this process of weathering; but it is by no means essential, for the northern faces of the walls of crevasses exhibit a well-developed superficial layer; and I have seen it even beneath huge boulders, where these were not in direct contact with the ice.
But one conclusion appears to me to be deducible from these experiments, and that is in perfect accordance with the results of ocular investigation. Glacier ice is essentially devoid of all pores, fissures and cavities, save the closed water-chambers; though of course, like all other brittle bodies, it is liable to become fissured and fractured by pressure from without. Fissures and cavities produced in this way, however, are accidental and not essential. But it is a remarkable feature of glacier ice, that it is liable to weather in a peculiar manner, becoming fissured and breaking up into irregular fragments to a certain depth. The superficial layer formed in this way is eminently porous, and absorbs fluids like a sponge.
In arriving at these results, however, I again regret to find myself in direct opposition to the current doctrine based on the statements of Prof. Agassiz, from whose 'Systeme Glaciaire' I continue my series of quotations.
(8) "Capillary fissures.The true capillary fissures are very different from the superficial fissures which have just been described (3). They exist not merely at the surface, but are found on the walls of crevasses and in the interior of cavities where the rays of the sun  never penetrate. They are larger than the little fissures which have just been mentioned, and far less numerous, particularly in the regions in which the latter abound. Their distribution is not uniform in the interior of the compact ice," p. 154; but (M. Agassiz goes on to explain) they are arranged in bands and zones, which, becoming more completely infiltrated with water than the intermediate ice, appear blue and transparent, and are the blue veins.
(9) "The quantity of bubbles with which the white ice is filled, is the reason why the fissures are more slowly propagated in it; the air, by its elasticity, being unfavourable to the formation of fissures (l'air qui est de sa nature elastique ne favorisant aucunement le crevassement). By degrees, however, and in proportion as the infiltration perpetuates itself, the rigidity increases, and the fissures multiply in proportion. Every bubble that a fissure meets in its course loses its aëriform contents. It becomes transparent, and the opacity of the mass is so far diminished. The consequence of this multiplication of fissures is continually to diminish the number of bubbles, and by this means to render the ice more and more transparent and blue.
"It will be evident to any one who has followed the progress of modern physics, that this phænomenon is due solely to the diathermanicity of ice. The air first and then the water becomes heated through the ice. However minute may be the degree of heat which is thus transmitted to them, it is enough to melt a part of the ice which surrounds them, and thereby to increase the cavity in which they are imprisoned. I do not think, however, that any very great importance should be ascribed to this phænomenon; and the fact that it is produced only when the ice is exposed directly to the rays of the sun, is in my eyes an indication that it exercises no notable influence on the mechanism of glaciers."P. 157.
(10) "When the ice has acquired a certain degree of transparency, and the network of capillary fissures is fully established in it, water and air penetrate into the fissures with great facility. One may assure oneself of this in many ways, among others by the following experiment, which I have repeated many times. Let a cube of ice of a few decimetres on the side be detached from the bottom of a crevasse, in that part of the glacier where the ice is most transparent, and placed upon a rock. At first, a few fissures will appear on the surface, then these fissures will be gradually propagated into the interior, and the network becoming more and more complex, will by degrees reach the base. If, then, the block of ice be turned upside down, and water be poured upon it, all the fissures will disappear from above downwards, in the same order as they were formed. The block will remain  perfectly transparent so long as it is saturated; but so soon as one leaves off watering, the fissures reappear where they last appeared when the block was reversed."P. 161.
(11) "The angular fragments are the consequence and the product of the capillary fissures. When a morsel of compact ice is exposed for some time to the air, it becomes decomposed into a certain number of angular fragments, which are the smaller the more numerous the fissures. The same thing would happen to the glacier if its thickness were less, and if the external heat had access to it on all sides. Nevertheless its surface decomposes more or less, the fissures dilate in consequence of the circulation, and the fragments are so dislocated as to be movable on one another without however becoming detached."P. 163.
(12) "The angular fragments and the capillary fissures seem to disappear the moment the ice is covered. Thus on sweeping clean a part of a moraine, or the side of a gravel cone, the ice beneath is found to be perfectly smooth, and apparently without a trace of a fracture. But it is sufficient to leave these same surfaces uncovered for some instants, and the capillary fissures immediately show themselves, and, in consequence, the angular fragments. They appear with such regularity, that one might be tempted to believe that they are formed spontaneously at the very moment of their appearance. But on examining them with a little attention, one becomes convinced that they are of older date.
"I by no means pretend to deny that heat, acting suddenly at the moment the moraine is uncovered, may not develope some cracks. I have myself seen such cracks form suddenly (par eclat), but I conceive they are but few. If it were otherwise, and if the fissures were formed as they appear, it would be necessary to suppose that there are none in the ice of the moraine before it is uncovered, which would be contrary to all we know of the transformations of the ice."P. 165.
(13) "Let us now make the opposite experiment, and cover with sand and gravel a portion of the surface of the glacier. However fissured and disaggregated it may be, the fissures and angular fragments will disappear at the end of some time so completely, that on removing the gravel the surface will be found as compact and transparent as that of a portion of moraine which has never been uncovered. And yet it is not probable that the fissures have reunited during the interval. It is, on the contrary, the gravel, which, intercepting the air and keeping the fissures full of water, renders them invisible, and gives to the whole mass a false appearance of compactness, which ceases the moment the air again has access to the fissures."P. 166.
 If the extract (8) were to be taken merely as a description of the superficial layer of a glacier, I should only have to object, that, so far as I have been able to observe, the colour of the disintegrated blue veins is not much affected by the water they contain, and that no amount of watery infiltration will confer on the white ice the beautiful transparency and colour of the blue veins. But Prof. Agassiz over and over again affirms that the whole substance of the glacier is traversed by capillary fissures, and his infiltration experiments are supposed by himself conclusively to demonstrate the fact. I must confess, however, that I have neither been able to observe what Prof. Agassiz supposes he has observed, nor, were our observations in unison, could I admit his explanations.
Take for instance the citation (9). How can the elasticity of the air-bubbles influence the formation of fissures in the continuous mass of rigid and eminently brittle ice which encloses them? How is the statement, that the ice becomes more rigid as the fissures are developed in it, these fissures being supposed to be filled with water, compatible with that made in (2), that this same water is the chief source of the plasticity and compressibility of ice?
Again, I am at a loss to understand the "diathermanicity" theory. Prof. Agassiz brings forward no experimental proof that air contained within ice is more heated by the sun's rays than the ice itself; and, a priori, it seems improbable that the more diathermanous body should be more heated than the less. It is true, I cannot pretend to have "followed the progress of modern physics;" but I am emboldened to say this much by the fact, that you, who have, seem to find at least equal difficulty in adopting Prof. Agassiz's explanation.
With regard to the experiments detailed in (l0), (12), and (13), it will be observed that my results in the main agree with those of Prof. Agassiz, if, as before, we confine ourselves to the superficial layer; but, as I have shown, it is an error to extend the conclusions drawn from the structure of the superficial layer to the deep ice. This, however, is what Prof. Agassiz has done; and it is curious to find him in (12) refusing to follow out a suggestion which would have led to the solution of his difficulty, because it is "contrary to all we know of the transformations of the ice." What do we know at present of the transformations of the ice?
It is important to remark again, that as regards matters of fact, Prof. Agassiz's statements with respect even to the deep ice are, so far as they go, not essentially different from mine. He admits (12) that no fissures are at first visible in the deep ice;had he taken the trouble to make the experiment, he would have found also that  coloured liquids cannot be made to enter it; and he admits that the establishment of a complete system of fissures through a block of ice, and its consequent permeability, are matters of time and exposure (l0). See also citation (1), and p. 289 of the 'Systeme Glaciaire.'
I omitted to make the experiment detailed in (13). It is singular that in (12) Prof. Agassiz states that "the angular fragments and the capillary fissures seem to disappear the moment the ice is covered," while in (13) the operation is said to take some time; but, supposing the fact to be as Prof. Agassiz says, it seems to me to be in the highest degree probable that the fissures have reunited during the interval. At any rate, I cannot admit Prof. Agassiz's explanation, for surely loose gravel is not exactly a substance calculated to "intercept air and keep fissures full of water."
It would take up too much space, and serve no useful purpose, to quote at length the account Prof. Agassiz gives of his infiltration experiments ('Syst. Glaciaire,' pp. 170-179). Those who will turn to the original, will find that they are all vitiated by the absence of any discrimination between the deep and the superficial ice, and between "capillary fissures" and accidental cracks. Not one of Prof. Agassiz's experiments affords the slightest evidence that capillary fissures are a primitive and essential constituent of the structure of the deep ice of a glacier.
The experiments of the Messrs. Schlagintweit (l. c. p. 12) appear to me to be equally inconclusive; these gentlemen, like Prof. Agassiz, having omitted to take the precaution of clearing away the superficial layer from the mouth of the cavity to be filled with the infiltration fluid. Unless this be done, the superficial layer sucks up the coloured liquid, which becomes diffused in the way they described. And if the cavity (as may readily happen, especially with such large ones as those employed by these experimenters) communicates by an accidental fissure with some other part of the surface of the glacier (say the wall of a crevasse, or the roof of such a cavity as Prof. Agassiz's infiltration gallery), it should be well remembered that the fluid which drains through will not run out in a stream from the termination of a crack, unless the superficial layer has been cleared away; otherwise, it will fill the fissures of the superficial layer and appear as a great patch. The observer then, seeing nothing but fissures full of coloured infusion at each end of the course of the fluid, naturally enough imagines that in its intermediate course the fluid has traversed similar fissures. This conclusion would be at once dissipated by cutting away the superficial layer and laying open the  infiltration cavity,a precaution which does not seem to have occurred to either Prof. Agassiz or the Messrs. Schlagintveit.
I will conclude with a few words upon the relation of structure to the arrangement of dirt upon the surface of a glacier. The great "dirt-bands" have never been proved to be connected with any peculiar structure of the ice on which they lie, and it has been shown that they may be the mere result of the influence of the motion of the glacier upon the form of any patch of dirt scattered accidentally upon its surface; but besides these "dirt-bands," the dirt on a glacier frequently presents a definite arrangement upon a smaller scale, which is connected with the minute structure of the glacier. We have both observed, for instance, in those parts of the Mer de Glace in which the structure is vertical, that the superficial layer of the wall of a crevasse is weathered into granules of tolerably even size and similar form. Nevertheless, dirt (or a coloured infusion) accumulates in larger proportion in those fissures which are parallel with the cleavage, and thus, from a little distance, the surface of the ice appears as if striated or ruled with lines parallel to the structure. The lines are separated by the width of the granules, and there may be several interposed between two blue veins.
Why it is that those intergranular fissures which are parallel with the cleavage are the larger, is a question I will not for the present attempt to answer. It may be that the weathering takes place more rapidly in this direction, or it may be that these fissures being in the course of the flow of the water produced by the superficial waste of the ice, become enlarged more rapidly than the others.
These markings, and the similar ones frequently to be observed on the upper surface of a glacier, might be termed "dirt-lines," to distinguish them from the great "dirt-bands." There is a third mode of arrangement of dirt, which, like the "dirt-lines," is dependent on the weathering of the ice, but the resulting striæ are broad streaks, and not mere lines. These may perhaps be termed "dirt-streaks."
I became acquainted with these quite recently, when, induced by Prof. Forbes's description and representation of the "structure" of the glacier of La Brenva, I paid a visit to that glacier. Prof. Forbes states,
"The alternation of bluish-green and greenish-white bands which compose this structure, gives to this glacier a most beautiful appearance as seen from the mule-road. An attempt has been made in plate 5 to give some idea of this most characteristic display, and which is better seen here than in any other glacier whatever with  which I am acquainted. The sketch was taken by myself from the point marked k on the map in July 1842."Travels, 2nd Ed., p. 203.
It must at once strike any one conversant with the ordinary character of the veined structure, that at the distance of the point on the mule-road from which Prof. Forbes's view is taken any veins of the usual dimensions must be totally invisible; and I therefore approached La Brenva with the desire, if not the hope, of making the acquaintance of glacier structure on a new and gigantic scale.
Viewed from the mule-path, or from the old moraine at the commencement of the pine wood celebrated by De Saussure, the lower part of the glacier of La Brenva exhibits numerous crevasses, which appear to run nearly parallel with its length, so that the icy mass is divided into a series of parallel crests or ridges. The lateral faces of these ridges form perpendicular cliffs of ice, and present dark stripes directed in a longitudinal and nearly horizontal direction; but where an end view of a ridge is obtained, the stripes run either horizontally and transversely (as in the more central parts of the glacier), or are curved up towards the sides (as in the more lateral parts).
These markings are evidently those described and faithfully figured, as the "structure" of the glacier, by Prof. Forbes; but I cannot say I should have called them bluish-green. They looked to me simply dark and dirty. But I should state, that the weather, when I visited the glacier, was wet and cloudy.
Nevertheless, on descending on to the glacier itself, I found its structure, though very beautifully developed, to be in nowise remarkable for the size of its veins, which varied in length from an inch to eight or nine feet, and in breadth from a fraction of an inch to nine or ten inches. Veins of the latter dimensions, however, were rare; the majority having a thickness of less than an inch. The lenticular form was very well marked, and the veins were commonly separated by less than their thickness of white ice. I need hardly say that these veins became indistinguishable at a very short distance.
The streaks so conspicuous a long way off, on the other hand, became less sharply defined as I approached, and at length showed themselves to be nothing more than accumulations of the fine dirtspread more or less over the whole cliff-like wall of ice,in streaks of four to ten inches in breadth, and of variable length. They ran parallel with one another and with the structure, at a distance varying from a few inches to six or seven feet; and they were entirely superficial, the dirt never extending deeper than the weathered superficial layer.
 It became clear, therefore, that the markings were neither structure nor stratification, but a peculiar kind of dirt-marks; and the next point was to ascertain the conditions of their formation.
On close examination, the face of the ice-cliff exhibiting these markings appeared to be worn into a sort of wavy or rippled surface, the length of the ripples having a general direction downwards. The close-set veins, on the other hand, traverse the face of the ice, as has been said, nearly horizontally. The whole surface of the ice is more or less dirty, not half-a-dozen square inches being without its little grains of sand and minute gravel, brought down, as I imagine, by the water which continually trickles from above; but the greater part of this impurity is invisible from a small distance, unless where it is specially accumulated.
Such accumulation takes place in two localities; in the first place, on the little shelves afforded by the upper and more southerly aspects of the "ripples" above referred to. Here the dirt accumulates quite independently of the structure, and as a consequence either of the form or of the aspect of the part on which it lodges.
From a little distance these aggregations appear as spots and patches, but further off they cease to be visible, and the glacier between the horizontal streaks appears white.
These streaks mark the second locality in which the dirt aggregates. Now whenever I carefully examined the surface of the glacier at these points, I found it to be weathered into large granules, separated by coarse fissures which extended for a considerable depth into the substance of the glacier; while the parts intermediate between the streaks, and which appear white from a distance, presented very much smaller granules, with fissures proportionately finer, and extending inwards for but a very small distance. In short, where the dark streaks existed, the ice was deeply weathered and coarsely granular, affording lodgment for dirt to a depth of two or three inches; while the intermediate substance had undergone only superficial weatherings and its finely granular structure afforded but little facility for the intrusion of foreign matters.
The "dirt-streaks," then, are due to the unequal weathering of the ice; but why does the ice weather unequally? On seeking for an answer to this question, I found that every dirt-streak corresponded either with a very large blue vein, or with a closer aggregation than usual of smaller blue veins, while the intermediate substance contained a preponderance of the smallest blue veins; so that the coarse granules were the result of the weathering of parts of the glacier, composed either exclusively, or for the most part, of blue ice, while those in  which the proportion of white ice was larger, weathered less deeply and into finer granules.
The markings of La Brenva, then, are neither ordinary dirt-bands nor direct expressions of structure, nor direct evidence of stratification, but they are produced by the more ready lodgment of dirt in some parts of the superficial layer of the glacier than in others, in consequence of the more coarse and deep weathering of these parts; which again, is the result of the predominance of blue ice over white in these localities.
Why blue ice should predominate at intervals in the substance of this glacier,whether the like alternation of structure holds good in glaciers generally,and whether it has any relation to a primitive stratification, are problems of great interest and well worthy of investigation.
With regard to the second, I will merely express a belief that some such alternation of structure does obtain in glaciers generally; for the appearances presented by good sectional views of glaciers, such as that exposed on the north side of the Allalein, are so similar to those exhibited by La Brenva, that I cannot doubt the identity of their cause. I had been in the habit of regarding the appearances referred to as direct evidences of stratification; but if my supposition be correct they will merely be evidences of an alternation of structure which may or may not depend on stratification.
1 The Messrs. Schlagintweit (Untersuchungen, p. 17) adopt Prof. Agassiz's views on this point, and with him regard the prescence of water as a local and partial phenomenon. 2 The superficial layer is particularly well described by the Messrs. Schlagintweit in their Untersuchungen über die physikalische Geographie der Alpen, 1850.
1 The Messrs. Schlagintweit (Untersuchungen, p. 17) adopt Prof. Agassiz's views on this point, and with him regard the prescence of water as a local and partial phenomenon.
2 The superficial layer is particularly well described by the Messrs. Schlagintweit in their Untersuchungen über die physikalische Geographie der Alpen, 1850.