ICE AND GLACIERS. 



137 



the lower layers of still colder snow, it again freezes ; 

 thus it is that the firn first becomes granular and ac- 

 quires the temperature of the freezing-point. But as the 

 weight of the superincumbent masses of snow continually 

 increases by the firmer adherence of its individual granules, 

 it ultimately changes into a dense and perfectly hard 

 mass. 



This transformation of snow into ice may be artificially 

 effected by using a corresponding pressure. 



We have here (Fig. 22) a cylindrical cast-iron vessel, 

 A A ; the base, B B, is 

 held by three screws, and 

 can be detached, so as to 

 remove the cylinder of ice 

 which is formed. After 

 the vessel has lain for a 

 while in ice-water, so as 

 to reduce it to the tem- 

 perature of 0, it is 

 packed full of snow, and 

 then the cylindrical plug, 

 C C, which fits the inner 

 aperture, but moves in it 

 with gentle friction, is 

 forced in with the aid of 

 an hydraulic press. The 

 press used was such that 

 the pressure to which the 

 snow was exposed could 

 be increased to fifty atmospheres. Of course the looser 

 snow contracts to a very small volume under such a 

 powerful pressure. The pressure is removed, the cylin- 

 drical plug taken out, the hollow again filled up with 

 snow, and the process repeated until the entire form is 

 filled with the mass of ice, which no longer gives way 



