1859.] 



on the Veined Structure of Glaciers, 



77 



traction ; and it is manifest that external pressure in this case assists 

 the heat, instead of opposing it, and that to fuse the metal, under pres- 

 sure, a less amount of heat will be necessary : hence the fusing point of 

 bismuth is lowered by the pressure. Now ice is a substance whicFi 

 behaves in this respect exactly like bismuth ; it contracts on being 

 liquefied, and if we subject it to pressure it will liquefy underlS2° Fahr. 

 — in its case also the melting point is lowered by pressure. Reference 

 was made to the theory of Mr. James Thomson, and the experiments 

 of Professor Wm. Thomson, which bore upon this question. 



Supposing then a prism of solid ice at 32*^ to be subjected to 

 pressure. This pressure reduces its melting point, say for the sake of 

 simplicity to 31° ; what must be the consequence ? The ice possesses a 

 temperature higher than its new melting point, and the excess of heat 

 which it possesses is applied to the liquefaction of a portion of it. 

 This effect was experimentally exhibited before the audience. A 

 prism of ice was placed between the surfaces of a small hydraulic pre^s, 

 and the prism was illuminated by the bright beam of an electric lamp. 

 The beam had been previously sifted of its heat by sending it through 

 a solution of alum, so that the light passed afterwards through the ice 

 without melting it. By means of a convex lens placed in front of it, 

 a magnified image of the prism of ice was cast upon a screen, and when 

 the pressure was gradually applied, lines were observed drawing them- 

 selves across the pressed mass in a direction at right angles to the 

 pressure : these lines the speaker had proved by strict examination to 

 be the edges of flat spaces in which the ice had been liquefied by the 

 pressure. 



The speaker's theory of the blue veins then was, that the glacier, 

 when subjected to intense pressure, also liquefied in flats perpendicular 

 to the direction of pressure : a means was thus provided for the expul- 

 sion of the air entangled in the ice, or in other words for the production 

 of veins containing less of air than the general mass of the glacier. A 

 portion of the water would be absorbed by the adjacent bubbled ice, 

 and refrozen when released from the pressure ; and the veined structure 

 would follow. 



The following experiment was referred to. A prismatic mass of 



ice was taken, the shape of which, in section, was ih&tofab c rf, fig. 1. 



