CONGELATION OF INFILTRATED WATER DISCUSSED. xix

times, and many of them during journeys, I believe that a substantial unity pervades the whole.

On only one point of any importance was the original theory, as delivered in the "Travels in the Alps," subsequently modified. In the earlier of my writings, I attributed the conversion of the powdery névé into the perfect glacier (as I believe nearly all preceding and contemporary authors had done), as well the alternations of perfect and porous ice in the " veined structure" of the latter (which at that time no one else had tried to account for), to the congelation by the winter's frost of water percolating through their interstices. In the former case, the

snowy granules; in the

interstitial spaces were those of the latter, the flaws due to the differential motion of the central and lateral parts of the glacier.

The congelation of infiltrated water was not a doctrine which originated with me, but rather one which I had assisted in banishing as much as possible from glacial speculations. Having endeavoured to demonstrate that no efficient congelation of this description can take place in summer when the motion of the glacier is most rapid, I would gladly have dispensed with it altogether. Still it was impossible to deny that the winter's cold must penetrate to some depth in the glacier, and at first I was willing to admit not only that it was the efficient cause of the conversion of the névé into ice, and of the glassy bands of the veined structure, but that it might by its expansion restore the level of the glacier, during winter, to the point from which it had fallen by the waste of the previous This was the idea I had in 1842.* But by the time of the publication of the first edition of my Travels in the Alps in 1843, I had already got rid of the necessity for allowing this vertical dilatation, by including the ascent of the glacial surface

summer.

* Fourth Letter on Glaciers (1842), p. 34 of this volume.

among the direct effects of plasticity.* I considered that my theory was strengthened by being rendered so far independent of an effect which I admitted with reluctance.†

There remained, however, three classes of facts requiring explanation, and which were evidently connected with one another (1) there was the reconsolidation of a glacier more or less extensively fissured by open cracks, which is yet seen ultimately to recover its continuity; (2) the reunion by pellucid ice of the surfaces rent by the bruising action producing the veined structure; (3) the transformation of the névé into perfect ice. On the first point I had early come to the conclusion that the greater fissures of ice were sealed up merely by the collapse and reunion of the particles influenced by time and pressure, and aided by the softening effects of the plentiful affusion of water. Next I was disposed to account, at least in part, for the reunion of the surfaces of internal sliding, or the formation of the veined structure when it arises or is modified in the glacier proper, by a similar result of pressure and cohesion. do not think that any recognition of infiltration in connection with this structure will be found in my writings subsequent to 1844. A third fact was, however, to be explained before the congelation of water in the depths of the glacier could be dispensed with ;—and until that was effected, it was, to say the least, superfluous to affirm that congelation had no effect in producing the "blue veins." The difficulty now centered on the conversion of the névé into glassy ice, a process (in my opinion) very intimately connected with the original formation of the veined structure in the higher glacier, as the following passage, written in Italy early in 1844 (therefore founded on my observations of at least the previous year), clearly shows:-After explaining the

*Travels, 1st Edit., p. 384; 2d Edit., p. 386.

† See the limitations mentioned in Travels, pp. 232, 360, 372 of both editions.

I

CONSOLIDATION BY FRICTION AND PRESSURE.

XX

effects of differential motion in developing the veined structure, it is added, "I believe that it is during the progress of the glacier thus subjected to a new and peculiar set of forces depending on gravity, and which remodel its internal constitution by substituting hard blue ice in the form of veins for its previous snowy texture, that the horizontal stratification observed in the higher part of the glacier or névé is gradually obliterated."* It will be seen in the pages of this volume that the identity of the process which (at least in the higher glacier) produces the blue veins of the ribboned structure, with the conversion of granular snow into glassy ice, remained for nearly three years a strongly fixed idea in my mind, but it only received a satisfactory development when I returned to Chamouni in 1846, thoroughly unsatisfied with the explanation of the conversion of the névé into ice by thaw and congelation, and determined, if possible, to find a better solution for it. In the meantime I avoided in my writings any farther allusion to the mode of this conversion, as to which I had merely sanctioned the traditional opinions adopted even by De Saussure, notwithstanding his well-founded objections to the dilatation theory.†

But in my autumn journey of 1846 this difficulty was removed, and I hastened, on my return, to record in my Thirteenth Letter on Glacierst my now clear conviction that all the phases of consolidation of a glacier are due to the effects of time and cohesion alone acting on a substance softened by the imminent approach of the thawing state, in opposition to the belief which I formerly, in common with most other persons, entertained, that snow could not pass into pellucid ice without

* Fifth Letter, p. 53 of this volume.

+ Ces mêmes neiges . . . abreuvées des eaux des pluies et des neiges fondus, se gelent pendant l'hiver, et forment ces glaces poreuses dont les glaciers sont composés."--Voyages, tom. i. § 527.

Reprinted in the present volume, page 199.

being first melted and then frozen.

Friction and pressure alone especially in the glacier,

I affirmed to effect the change, which, during a great part of the year, is kept on the very border of thawing by the ice-cold water which infiltrates it. In this condition, molecular attachment I stated to be comparatively easy, the opacity disappearing as optical contact is attained. The "glacification" of the névé takes place by the kneading or working of the parts under intense pressure, and the multitudinous incipient fissures are reunited by the simple effects of time and cohesion. Thus the conversion into ice is simultaneous, and in this case identical with the formation of the blue bands, which are formed where the pressure is most intense, and where the differential motion is a maximum, that is, near the walls of the glacier.

Thus the whole phenomena of the transformation of the glacier from fresh fallen snow into laminated cohering ice, which constitutes the character of the true glacial substance, even at great depths, were accounted for without the embarrassing admission of the penetration of the winter frosts to an unlimited extent.†

It was important that I should specifically point out this change in my opinion, of which the record is to be found in these pages, because it is the only one (I hope) which occasions a discrepancy of any consequence between my earlier and later

These expressions are verbally taken from the Thirteenth Letter, pp. 200, 201. Since a certain amount of congelation of infiltrated water must necessarily take place during winter, at least near the surface of a glacier, it is easy to see that it is a matter of nice discrimination to limit precisely its agency in a theory of glaciers. Certain phenomena cannot be produced without it. Such are the lenticular frozen cavities which were described by myself on the Glacier of Bossons, and which have been since particularly noticed by Messrs. Tyndall and Huxley, and by Mr. Ball. I have referred to them more than once in the papers printed in this volume, where I have described them as the limit of the veined structure when the dislocating forces are very great and the lateral compression small, and I have also

M. PERSON'S AND MR. FARADAY'S OBSERVATIONS.

xxiii

writings reprinted in this volume. It will also, I think, be admitted on examination, that the new doctrine preserved all that was sound and important in the old one, and added a new feature not only to the explanations given by myself, but to what was true in the writings of De Saussure and other older authors.

In 1850 I noticed M. Person's interesting observation deduced from his own experiments and those of M. Regnault, that the dissolution of ice is a gradual, not a sudden process, and so far resembles the more tardy liquefaction of fatty bodies, or of the metals which absorb their latent heat by degrees, and pass through intermediate stages of softness or viscosity. I hastened to avail myself of this new confirmation of views which my observations on glaciers had suggested. In like manner, had Mr. Faraday's fact of the congelation of water placed between two plates of ice, even though that ice be exteriorly melting, been noticed by me when it was first published, I would have unquestionably quoted it as a valuable auxiliary in explaining the possible congelation of water in the minuter fissures of the glacier, although since the change of views respecting the consolidation of the glacier mentioned above as adopted by me in 1846, the congelation of water in the crevices of the glacier ceased to be essential to the mechanical explanation of the movement and structure of the ice. The function of the infiltrated water seems to be that of preserving the whole ice in that state of softness which immediately precedes its dissolution, as well as of conveying hydrostatic pressure; and the cohesion of

(pp. 90, 162) pointed out a similar accident in lavas. The width of the longitudinal fissures in question shows that no lateral pressure was exerted sufficient to bring their bounding surfaces into contact, and that they could not have been filled with pellucid ice by any process short of the prolonged action of external cold upon infiltrated water. I believe, however, that these appearances are confined to the neighbourhood of the surface and sides of glaciers.