The Theory of Glacier motion. yy 



which it descends through the glacier. Wherever congela- 

 tion does take place, the capillary pores must be filled up, 

 and where it does not, the percolating water must proceed 

 till it meets with the larger fissures, through which it will 

 descend freely to the bottom of the glacier." 



Forbes's refutation of the theory of dilatation was two- 

 fold. He replied to it on a priori and on experimental 

 grounds. Thus he says, " The dilatation theory is founded 



on a mistake as to a physical fact The maximum 



temperature which a glacier can have, observes M. de 

 Charpentier {Essai^ pp. 9 and 104), is 0° centigrade or 

 32 Fahr., and the water in its fissures is kept liquid only by 

 the small quantity of heat which reaches it from the surface 

 water and the surrounding air. Take away this sole cause 

 of heat, i.e.^ let the surface be frozen and the water in the 

 ice must congeal. Now this is a pure fallacy ; for the fact 

 of the latent heat of water is entirely overlooked. The 

 latent heat of water expresses the fact that where that fluid 

 is reduced to 32° it does not immediately solidify, but that 

 the abstraction, not of a small quantity but a very large 

 quantity indeed, is necessary to convert the water at 

 32° into ice at 32°. Not a great deal less heat must be 

 abstracted than the difference between the heat of boiling 

 water and that at common temperatures. The fallacy, then, 

 consists in this : Admitting all the premises, the ice at 32° 

 (it is allowed that in summer during the period of infiltration 

 it cannot be lower) is traversed by fissures extending to a 

 great depth (for otherwise the dilatation would be only 

 superficial) filled with surface water at 32°. Night 

 approaches, and the surface freezes, and water ceases to be 

 conveyed to the interior. Then, says the theorist, the 

 water already in the crevices and fissures of the ice, and in 

 contact with ice, instantly freezes. Not at all ; for where is 

 it to deposit the heat of fluidity, without which it cannot 

 under any circumstances assume the solid form ? The ice 



