be frozen throughout. And that it is not [so frozen], the striking testimony of the continued stream of water issuing all winter from under the ice can hardly fail to convince us; still more, the circumstance mentioned in my Fourteenth Letter, that even in the month of February the source of the Arveiron becomes whitish and dirty as in summer, before a change of weather, proving (as I have there remarked) that "in the middle of winter a temporary rise of temperature over the higher glacier regions (which is the precursor of bad weather) not only produces a thaw there, but finds the usual channels still open for transmitting the accumulated snow-water."*

It thus appears quite certain that ice, under the circumstances in which we find it in the great bulk of glaciers, is in a state more or less softened even in winter; and that, during nearly the whole summer, whilst surrounded by air above 32°, and itself at that temperature, it has acquired a still greater degree of plasticity, due to the latent heat which it has then absorbed.t

I have mentioned that the observations of this and some previous summers have enabled me to extend the survey of the valley of Chamouni beyond the limits to which my Map was originally confined. I have also obtained a great number of approximate altitudes of all the highest summits of the chain of Mont Blanc, from the extended base which the distance from the Mont Breven to the Croix de Flégère (above 15,400 feet) has afforded me. But the results are as yet only partially calculated. I have also made some additions to our knowledge of the geography of the eastern part of the chain of Mont Blanc, by examining the Glacier of La Tour in its whole extent, which proved the configuration of the mountains to be different from what has been represented on all the maps and models which I have seen. The Glaciers of Argentière and.

*[See p. 133, note, of this volume.]

[See note to Philosophical Transactions, 1846, p. 209 (p. 166 of this volume), and also the paper following this one.]

1850.]

PASS OF THE COL DU TOUR.

227

Le Tour are separated throughout by a rocky ridge, but the Glaciers of Le Tour and Trient all but unite at their highest parts, and the main chain is prolonged with scarcely a break in the north-east direction, sending off only a spur towards the Col de Balme, which, perhaps from being the political boundary of Savoy and Switzerland, has been represented generally on an exaggerated scale. What surprised me most, was the great elevation of the axis of the chain at the head of the Glaciers of La Tour and Trient. I found it barometrically to be 4044 feet above the châlet of the Col de Balme, which, from five comparisons made with the observatory at Geneva, is 7291 English feet, or 2220 metres above the sea, a result agreeing closely with the recent measurement by M. Favre, which is 2222 metres. Adding this result to the former, we obtain 11,335 English feet for the height of the granitic axis at the lowest point between the Glaciers of La Tour and Salena on the side. of the Swiss Val Ferret. By a single direct barometrical comparison with Geneva, I obtained 11,284 English feet above the sea, or 140 feet higher than the Col du Géant. I was successful in traversing the Glacier of Salena to Orsières the same day, a pass which has not before been described, and which has this interest, in addition to the singular wildness of the scenery, that it includes those regions of beautiful crystallized protogine, here in situ, which have been known to geologists hitherto chiefly from the numerous moraines which they form in the valleys of Ferret and of the Rhone, and especially the majority of the blocks of Monthey, which have been derived, according to M. de Buch, entirely from this region of the Alps.*

[See a fuller account of this pass in my account of "Norway and its Glaciers," etc., and also in the "Tour of Mont Blanc," etc., p. 301.]

XX. ON SOME PROPERTIES OF ICE NEAR ITS MELTING POINT.*

During the last month of March I made some experiments on the properties of ice near its melting point, with particular reference to those of Mr. Faraday, published in the Athenæum and Literary Gazette for June 1850,† to which attention has been more lately called by Dr. Tyndall and Mr. Huxley in relation to the phenomena of glaciers.

Owing to indisposition, I have been obliged to leave my experiments for the present incomplete. But I am desirous, before the session of the Royal Society closes, to place on record some facts which I have observed, and also some conclusions which I deduce from these and other recent experiments and discussions.

Mr. Faraday's chief fact, to which the term "regelation has been more lately applied, is this, that pieces of ice, in a medium above 32°, when closely applied, freeze together, and flannel adheres apparently by congelation to ice under the same circumstances.

1. These observations I have confirmed. But I have also found that metals become frozen to ice when they are surrounded by it, or when they are otherwise prevented from transmitting heat too abundantly. Thus a pile of shillings being laid on a piece of ice in a warm room, the lowest shilling, after becoming sunk in the ice, was found firmly attached to it.

2. Mere contact without pressure, is sufficient to produce these effects. Two slabs of ice, having their corresponding surfaces ground tolerably flat, were suspended in an inhabited room

From the Proceedings of the Royal Society of Edinburgh. 19th April 1858. [The abstract of this communication by Mr. Faraday, so far as it relates to the properties referred to above not being otherwise conveniently accessible, is reprinted in Appendix II. to the present volume.]

1858.] GRADATION OF TEMPERATURE IN FREEZINg water. 229

upon a horizontal glass rod passing through two holes in the plates of ice, so that the plane of the plates was vertical. Contact of the even surfaces was obtained by means of two very weak pieces of watch-spring. In an hour and a half the cohesion was so complete, that, when violently broken in pieces, many portions of the plates (which had each a surface of 20 or more square inches) continued united. In fact, it appeared as complete as in another experiment where similar surfaces were pressed together by weights. I conclude that the effect of pressure in assisting "regelation" is principally or solely due to the larger surfaces of contact obtained by the moulding of the surfaces to one another.

3. Masses of strong ice, which had already for a long time been floating in unfrozen water-casks, or kept for days in a thawing state, being rapidly pounded, showed a temperature of 0°-3 Fahrenheit below the true freezing point, shown by delicate thermometers (both of mercury and alcohol), carefully tested by long immersion in a considerable mass of pounded ice or snow in a thawing state.

4. Water being carefully frozen into a cylinder several inches long, with the bulb of a thermometer in its axis, and the cylinder being then gradually thawed, or allowed to lie for a considerable time in pounded ice at a thawing temperature, showed also a temperature decidedly inferior to 32°, not less, I think, than 0.35 Fahrenheit.

I think that the preceding results are all explicable on the one admission, that Person's view of the gradual liquefaction of ice is correct (Comptes Rendus, 1850, vol. xxx. p. 526),* or that ice gradually absorbs latent heat from a point very sensibly lower than the zero of the centigrade scale.

* Quoted by me in 1851, in my sixteenth letter on Glaciers [p. 225 above].— [See also the anticipation of the fact in my paper of 1846, note to page 156 of this volume. Even in 1851 I was unacquainted with Mr. Faraday's experiment, the publication of which was, so far as I know, confined to the journals quoted at page 228. As I was in Germany at the time, they naturally escaped my notice. To facilitate reference to Mr. Faraday's observation, I have, as already mentioned, thought it well to reprint the report of it in the Appendix.]

I. This explains the permanent lower temperature of the interior of ice.

Let AB be the surface of a block of ice contained in water

at what is called a freezing temperature. That temperature is marked by the level of the line QP above some arbitrary zero. LM is, in like manner, the permanent but somewhat lower temperature possessed by the interior of the ice. The space, partly water, partly ice, or partaking of the nature of each, MNOP, has a temperature which varies from point to point, the portion NO corresponding to what may be called the physical surface of the ice between AB and ab, which is "plastic ice," or "viscid water," having the most rapid variation of local temperature.

II. Such a state of temperature, though it is in one sense permanent, is so by compensation of effects. Bodies of different temperatures cannot continue so without interaction. The water must give off heat to the ice, but it spends it in an insignificant thaw at the surface, which therefore wastes even though the water be what is called ice cold, or having the temperature of a body of water inclosed in a cavity of ice.*

* I incline to think that water, in these circumstances, may, though surrounded by ice, have a fixed temperature somewhat higher than what is called 32°. But I have not yet had an opportunity of verifying the conjecture.

(My idea is, that the invasion of cold from the surrounding ice is spent in producing a very gradual “regelation" in the water which touches the ice, leaving the interior water in possession of its full dose of latent heat, and also of a temperature which may slightly exceed 32o. By similar reasoning, a small body of ice, inclosed in a large mass of water, will preserve its proper internal temperature below 32°; but, instead of regelation taking place, the surface is being gradually thawed. This is the case contemplated in the paragraph of the text to which this note refers.) N.B.-The words in brackets were added to this note during printing. 13th May 1858.