ICE AND GLACIERS 243 



On the other hand, when we withdraw more heat from 

 the mixture of ice and water, the water gradually freezes; 

 but as long as there is still liquid water, the temperature 

 remains at zero. Water at o has given up its latent heat, 

 and has become changed into ice at o. 



Now a glacier is a mass of ice which is everywhere inter- 

 penetrated by water, and its internal temperature is there- 

 fore everywhere that of the freezing-point The deeper 

 layers, even of the fields of neve, appear on the heights 

 nrhich occur in our Alpine chain to have everywhere the 

 same temperature. For, though the freshly fallen snow of 

 these heights is, for the most part, at a lower temperature 

 than that of o, the first hours of warm sunshine melt ita 

 surface and form water, which trickles into the deeper 

 colder layers, and there freezes, until it has throughout been 

 brought to the temperature of the freezing-point. This 

 temperature then remains unchanged. For, though by the 

 sun's rays the surface of the ice may be melted, it cannot 

 be raised above zero, and the cold of winter penetrates as 

 little into the badly conducting masses of snow and ice as 

 it does into our cellars. Thus the interior of the masses of 

 neve, as well as of the glacier, remains permanently at the 

 melting-point. 



But the temperature at which water freezes may be altered 

 "by strong pressure. This was first deduced from the me- 

 chanical theory of heat by James Thomson of Belfast, and 

 almost simultaneously by Clausius of Zurich; and, indeed, 

 the amount of this change may be correctly predicted from 

 he same reasoning. For each increase of a pressure of one 

 atmosphere the freezing-point is lowered by the i-ii5th part 

 of a degree Centigrade. The brother of the former, Sir W. 

 Thomson, the celebrated Glasgow physicist, made an experi- 

 mental confirmation of this theoretical deduction by com- 

 pressing in a suitable vessel a mixture of ice and snow. 

 This mixture became colder and colder as the pressure was 

 increased, and to the extent required by the mechanical 

 theory. 



Now, if a mixture of ice and water becomes colder when 

 it is subjected to increased pressure without the withdrawal 

 of heat, this can only be effected by some free heat be- 



