From James David Forbes¹

XXXI. Remarks on a paper “On Ice and Glaciers” in the last Number of the Philosophical Magazine. In a letter to Prof. Tyndall. By Prof. J. D. FORBES.

MY DEAR SIR,

IN the part of your short paper “On Ice and Glaciers,” in the Philosophical Magazine for February,² in which my writings are referred to, I think that my meaning has been misapprehended. Perhaps I can very briefly state what, in each of two cases, I conceive to be a needless source of difficulty unintentionally entertained by you with reference to my theoretical views or explanations.

The explanations which I now offer will tend, I hope, towards a reconcilement of our conclusions, and not to controversy.

I. With reference to the theory of “Regelation,”³ you say, on p. 98 of the Phil. Mag. For February, —“Why is the ‘rapid pounding’ necessary in the experiment of Prof. Forbes? Doubtless in order that the ice may be brought into contact with the thermometer before its temperature has risen to 32°.⁴ But give the ice time to rise to 32°; let its last residue of cold be abolished—the mass thus warmed, and in which the finest thermometer will not show the smallest fraction of a degree below 32°, may, with the utmost facility, be converted by pressure into solid ice.”

And again, “Let the thawing surface of a mass of ice be scraped away, so as to obtain a fine ice-powder possessing the temperature of that surface. Let not the alleged magazine of cold within the ice be at all called upon; such a powder, or more properly fine slush, the temperature of which no thermometer can show to be below 32°, may, as in the former case, be converted by pressure into solid slabs of ice.”

According to my view, in the former of these cases the thermometer can only show the temperature of the ice when the latter is freshly pounded. If kept at a thawing temperature, each icy fragment becomes invested with a film of water which has the temperature of water, not of ice. You say, “Give the ice time to rise to 32°.” I answer, that it is not in the nature of ICE to do so. The heat communicated becomes latent in the water produced, not one iota of it reaching the ice, which, whilst it remains Ice, is incapable of having its temperature raised. Precisely the same consideration applies to the second paragraph. Granulate the ice as we will, whilst any Ice remains, it has the temperature of Ice, not of Water. The powder or fine slush which you describe has the temperature of 32°, because the thermometer is in contact with the water alone, which forms so large a part of is composition. A thermometer is, in short, incapable of taking the temperature of ice unless that ice be dry. What, then, is the effect of the pressure which you describe as converting the “slush” into “solid slabs”? You admit, with me, that pressure is not necessary in the general case for regelation (p. 94). The effect of the pressure, then, is merely to banish nearly all the perfect water (that whose temperature the thermometer had shown) from the mass, leaving merely films of water (more or less viscid) between the still icy particles which constituted the slush. These particles, small as they are, contain each its magazine of cold, and being numerous as they are small, this cold is adequate to the consolidation of the interposed films of water, precisely as when the experiment is made on larger masses.

In this explanation I admit your view, that water is absolutely frozen in the process; but I believe also in the cohesive aggregation or welding under pressure of surfaces of ice softened by imminent thaw, though not yet reduced to water.

I think that, on consideration, it will appear clear to you that the preceding explanations render the facts you mention inevitable deductions from the doctrine that ice absorbs latent heat gradually*.

II. As regards the theory of the Veined Structure of Glaciers⁵ (p. 94 of your paper), there would perhaps be some inconvenience in my dictating the interpretation which certain passages of my writings more than twelve years old must bear, and declaring that no other interpretation is admissible. Such an affidavit of a pleader in his own cause might be received with distrust. It is better that others should study what I have written, and that they should conclude my meaning from an impartial examination of the whole documents. To facilitate this examination, I am about to publish, in a collected form, all my minor papers on the Theory of Glaciers⁶ which can throw any light on this question; and I hope that then, when the difficulty of fairly and fully examining what I have said on the subject has been removed, my meaning will be found to be both definite and intelligible.

In the present instance, if, as stated in your paper, more than one writer has attached a certain significance to the terms of my Thirteenth Letter on Glaciers⁷—and if, of these writers, Professor William Thomson be one, —it is not, I think, too much to affirm that it is probable that their concurring, though independent, inference as to my meaning is correct. I have not myself any doubt of its correctness; but I repeat that I do not cite myself as a witness in a case of interpretation which affects my own credit. Professor Thomson, or some one who maintains the same opinion which he does, will, I have no doubt, be ready and willing to give grounds for his judgement.

Why I very reluctantly appear in the matter is for this reason—that I believe you have not altogether understood, perhaps have overrated, what I claim to have established, particularly in connexion with the change admitted by all parties to have occurred in some of my ideas, in consequence of my journey in 1846,⁸ described in the Thirteenth Letter before mentioned. Several passages at pages 95 and 96 of your paper seem definitely to indicate that you ascribe to me an abandonment of a previously elaborate theory of the cause of the Veined Structure, and the substitution of a theory subversive of, and superseding in all points the old one. I will cite only one passage, including a quotation from myself, as given at p. 95 of your paper. It is as follows:—

“‘Mr. Hopkins,’ writes Professor Forbes in 1845, in reference to an experimental proof, ‘denies that the ribboned (veined) structure is produced by differential motion….. No person who has seen the model made, or even been told how it was made, and inspects the ribboned structure upon its surface, can, I think, unless influenced by previous theoretical views, entertain any other opinion.’ Is it to be supposed that convictions thus strongly uttered, based upon years of observations, and established, according to the above quotation, by the testimony of the senses themselves, are meant to be reversed by a single observation, which, after all, is essentially defective, involving, in reality, not a fact, but an opinion?”

The italics are yours. Here I am represented as stating a theory of the veined structure in 1845, which, it is argued, I could not (as I am represented to have done) have relinquished in merely one or two sentences of a letter of 1846. I shall probably give you satisfaction when I assure you that the doctrine of the origin of the veined structure, so far as described in the preceding citation, and in several other passages of those pages of your article already referred to (pp. 95 and 96), was neither given up by me in 1846, nor in any subsequent year—nay, that I hold it still. I also entertain the very same views with reference to Mr. Darwin’s theory of banded lavas which I did in 1845,⁹ which at page 96 you understand me to have abandoned, or at least that I am represented to have abandoned them, in 1846*.

The difficulty, which until 1846 did not become thoroughly plain to me, was this:— How is the structure of glassy ice induced in the glacier without previous fusion and fresh congelation? In 1842 I believed that the plates of hard blue ice, which by their interposition between spaces of a less compact character, compose the Veined Structure, could only be due to the freezing of water in the glacier. I considered them as veins of infiltrated water frozen during winter. To the same cause I then attributed the conversion of the granular mass of the névé¹⁰ into glassy ice. It was always a reluctant admission on my part: for the congelation of infiltrated water to any great extent, even in winter, was a relic of the Dilatation theory of glacier motion;¹¹ and against that theory I had entered an earnest protest. Between the printing of my Fourth Letter on Glaciers in 1842,¹² and the publication of my ‘Travels’ in 1843,¹³ I had already abandoned the use of this reluctantly admitted fact—the winter congelation of the depths of the glacier—so far as I had employed it to explain the recovery of the level of the glacier between autumn and the following spring; and I felt my position to be stronger when I obtained another explanation*. A similar but more effectual deliverance from a felt difficulty arose in 1846, when I arrived at a clear persuasion of what (as will appear form a comparative study of my writings) had already dawned upon me some time previously, namely, that the glassy structure of ice is attainable by the cohesion under pressure (especially if aided by motion with friction, or kneading) of the semi-opake¹⁴ and porous material of the glacier.

Although I had previously expressed several times my growing belief that the renewed cohesion of the bruised surfaces of the ice in the glacier proper, under the mutual pressure of their parts, might account for the facts both of motion and of structure there, I had not until the summer of 1846 disembarrassed myself of the complication arising from the conversion of granular snow in the névé into pellucid ice. I had, indeed, come very near to it; for already, in 1844, I had approximated the two phænomena in the following passage, in which, after speaking of the differential motion which tends to produce the veined structure in the glacier proper, I added — “I believe that it is during the progress of the glacier thus subjected to a new and peculiar set of forces depending upon 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*.” You see that even at this date the formation of the hard blue ice of veined structure, and the conversion of the névé into pellucid ice, appeared to me to be very intimately connected, —so much so, that until the last could be explained without interior congelation, little was gained in point of simplification by rejecting it in the former instance. It will be found, as I have said, by a consecutive perusal of what I wrote between 1844 and 1846, how gradually I approached the conviction (only obtained on the spot in the latter year) that the phænomena might be all explained by pressure and cohesion, the latter arising from the softening of snow or ice when thaw is imminent or in progress. In 1846, then, I abandoned no part of the theory of the veined structure on which, as you say, so much labour had been expended, except the admission, always yielded with reluctance, and got rid of with satisfaction, that the congelation of water in the crevices of the glacier may extend in winter to a great depth†.

I remain, dear Sir, | Yours faithfully, | JAMES D. FORBES.

Prof. Tyndall, | Royal Institution.

P.S. —It is foreign to the purpose of this letter to express an opinion as to whether any theory which you have formed as to the “veined structure” is independent, or otherwise, of the fundamental idea of differential motion arising under conditions of unequal pressures combined with friction. I reserve it until your matured views are published. But it did appear to me, some time ago, that in Mr. Sorby’s most ingenious researches on slaty cleavage,¹⁵ and in your extension of them to homogeneous bodies such as wax and dough, the efficiency of differential motion was tolerably present. —J. D. F.

[* See Mr. Faraday’s remarks on Regelation in our present Number, p. 162, —ED.]¹⁶

[* For the sake of clearness, I hope I may be permitted to state that the assumption to which I referred is expressed by Mr. Darwin in the following words:— “In the ice the porous laminæ are rendered distinct by the subsequent congelation of infiltrated water;”¹⁷ and my statement is, that Prof. Forbes does not use a word which would lead us to suppose that he wished to modify that assumption.¹⁸ —Phil. Mag. 1845, vol. xxvi. p. 354¹⁹ . —J. T.]

* ‘Travels,’ 1st ed. (1843), p. 384, or 2nd ed. p. 386; and note the limitations under which congelation in the interior of the glacier is admitted, at pages 232, 360, 372 of both editions. In so far as the glacification of the névé is concerned, it was no assumption of mine. I accepted the universal opinion of the time as stated by De Saussure, as well as by MM. de Charpentier and Agassiz.

* Sixth Letter on Glaciers, 1844.²⁰

† That this is a question of some nicety will be readily perceived; and my caution in overtly rejecting its influence altogether may be understood. For winter congelation must act to some depth; and certain phænomena cannot be explained without it. Such are the lenticular frozen cavities which you have described in your earliest paper on glaciers, which I take to be the same as those longitudinal infiltrated cracks previously described by me as visible in the glacier of Bossons,²¹ and other torrential glaciers. Such icy infiltrations are apparently too wide to be produced except by direct congelation. But then they are always near the side or surface of a glacier.

J. D. Forbes, ‘Remarks on a paper “On Ice and Glaciers” in the last Number of the Philosophical Magazine. In a letter to Prof. Tyndall. By Prof. J.D. Forbes’, Phil. Mag., 17 (1859), pp. 197–201.