150 Professor Forbes' s Thirteenth Letter on Glaciers. 



disappearing, as the subjoined note, on the very competent 

 authority of M. Gay Lussac, shews.* The crystalline forces 

 act on the snowy granules when brought into close contact 

 by pressure, and the imprisoned air is then distributed in the 

 direction of the lines of tearing, in the form of layers of regu- 

 lar globules, just as in the case of the banded lavas which 

 have been so well described by Mr Darwin.t Bishop Rendu, 

 whom I had the pleasure of visiting at Annecy, remarked 

 a familiar circumstance which illustrates the same thing. 

 We often see, in the coldest weather, that opaque snow is 

 converted into translucent ice by the sliding of boys on its 

 surface ; friction and pressure alone, without the slightest 

 thaw, effect the change, which must take place still more 

 readily in the glacier, where the mass is, during a great part 

 of the year, kept on the very border of thawing, by the ice- 

 cold water which infiltrates it. In this condition, molecular 

 attachment amongst the granules must be comparatively 

 easy, and the opacity disappears in proportion as optical con- 

 tact is attained. Most evidently, also, the icy structure is 

 first induced near the sides of the glacier where the pressure 

 and working of the interior of the ice, accompanied with in- 

 tense friction, comes into play, and the multitudinous inci- 

 pient fissures occasioned by the intense strain, are reunited 

 by the simple effects of time and cohesion. J 



* " II n'est plus permis aujourdhui d'avoir une foi aveugle au principe si 

 banalement repete des anciens Chimistes, corpora non agunt nisi soluta. II est 

 certain, au contraire, que tous les corps, solides, liquides, et aeriformes, agissent 

 les uns sur les autres, mais que, des trois etats des corps, I'etat solide est le 

 moins favorable a I'exercice de I'affinite." — Annales de Chimie et de Physique, 

 Juin 1846, p. 231. 



t On Volcanic Islands, and in Philosophical Magazine, April 1845. 



X A very remarkable peculiarity is observed in some of the glaciers of 

 Switzerland, which distinguishes them from the more rapid and precipitous 

 ones of Savoy. The glaciers of the Aar, Rhone, and Great Aletch, exhibit a 

 degree of crystalline structure which I have nowhere else observed ; broad, la- 

 minated, crystalline plates (not unlike, in general size and polish, to those of 

 hypersthene in hypersthene rock), shew a development of crystallizing force 

 which must evidently be the effect of long time, and probably of comparatively 

 slow motion. The reflection of the sun or moon-beam from these plates gives 

 to the glaciers I have mentioned a dazzling effect, which I have not observed on 

 the glaciers of the Pennine Chain. 



