516 THE LIFE OF JAMES 1). FORHES. [CHAP. 



long as there is any ice unmelted. By the applied heat 

 ice at zero is thus converted into water at zero, while 

 no observable rise of the thermometer takes place. Phy- 

 sicists say, therefore, that the applied heat has become 

 latent, and that water at zero contains a certain amount 

 more of latent heat than ice at the same temperature. 

 Again, when we abstract heat from the mixture of ice 

 and water, the water begins to freeze, but its temperature 

 remains at zero as long as there is any of it unfrozen. 

 Here water at zero has given off its latent heat, and has 

 become ice at zero. 



' Now a glacier is a mass of ice through which little 

 rills of water are running, and which, therefore, has 

 everywhere in its interior the temperature of the freezing 

 point. Even the lower layers of the nev6 appear, at such 

 heights as we find in the chain of the Alps, to have this 

 same temperature throughout. For, even though fresh 

 fallen snow at such elevations is generally colder than 

 zero, the first hours of warm sunshine melt its surface 

 and form water, which trickles into the lower cold layers, 

 and continues to freeze in them until they are by degrees 

 brought to the freezing point. This temperature thence- 

 forth remains unchanged. For by warm sunshine the 

 surface of the ice can be melted, but not raised above 

 zero, and the winter's cold does not penetrate deep into 

 the badly conducting snow and ice masses. Thus the 

 interior of the nev, like that of the glaciers, retains 

 unchanged the temperature of the freezing point. 



'But the temperature of the freezing point of water 

 can be altered by great pressure. This was first pointed 

 out by James Thomson of Belfast, and almost at the 

 same time by Clausius of Zurich, 1 as a consequence of 

 the dynamical theory of heat : and by the same reason- 



1 Prof, Clausius has habitually claimed so much that he sometimes 

 as in the present instance, gets credit for things he has himself ex- 

 pressly disclaimed. In his paper on this question (Pogg. Ann., 

 September 1850), he declares his object merely to be to show that 

 Thomson's result (Trans, ft. S. E. 1849) is in accordance with his own 

 mode of viewing the subject. P. G. T. 



