ICE AND GLACIERS. 119 



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

 at zero. Water at has given up its latent heat, and has 

 become changed into ice at 0. 



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

 penetrated by water, and its internal temperature is therefore 

 everywhere that of the freezing-point. The deeper layers, even 

 of the fields of neve, appear on the heights which 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 0, the first hours of 

 warm sunshine melt its 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 mechanical 

 theory of heat by James Thomson of Belfast, and almost simul- 

 taneously by Clausius of Zurich ; and, indeed, the amount of 

 this change may be correctly predicted from the same reasoning. 

 For each increase of a pressure of one atmosphere the freezing- 

 point is lowered by the T-r5^h part of a degree Centigrade. 

 The brother of the former, Sir W. Thomson, the celebrated 

 Glasgow physicist, made an experimental confirmation of this 

 theoretical deduction by compressing in a suitable vessel a mix- 

 ture 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 becoming latent; 

 that is, some ice in the mixture must melt and be converted 



