134 ICE AND GLACIERS. 



where 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 simultaneously 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 ^-j- th 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 suit- 

 able 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 becoming latent; that is, some ice in the 

 mixture must melt aiad be converted into water. In 

 this is found the reason why mechanical pressure can 

 influence the freezing-point. You know that ice occu- 



