118 ICE AND GLACIERS. 



philosophers. I will at once admit that philosophers themselves 

 were not a little perplexed by these results of their investiga- 

 tions. But the facts were there, and could not be got rid of. 

 How this mode of motion originated was for a long time quite 

 enigmatical, the more so since the numerous crevasses in glaciers 

 were a sufficient indication of the well-known brittleness of ice; 

 and as Tyndall correctly remarked, this constituted an essential 

 difference between a stream of ice and the flow of lava, of tar, 

 of honey, or of a current of mud. 



The solution of this strange problem was found, as is so 

 often the case in the natural sciences, in apparently recondite 

 investigations into the nature of heat, which form one of the 

 most important conquests of modern physics, and which constitute 

 what is known as the mechanical theory of heat. Among a 

 great number of deductions as to the relations of the diverse 

 natural forces to each other, the principles of the mechanical 

 theory of heat lead to certain conclusions as to the dependence 

 of the freezing-point of water on the pressure to which ice and 

 water are exposed. 



Every one knows that we determine that one fixed point of 

 our thermometer scale which we call the freezing-point or zero 

 by placing the thermometer in a mixture of pure water and ice. 

 Water, at any rate when in contact with ice, cannot be cooled 

 below zero without itself being converted into ice ; ice cannot 

 be heated above the freezing-point without melting. Ice and 

 water can exist in each other's presence at only one temperature, 

 the temperature of zero. 



Now, if we attempt to heat such a mixture by a flame 

 beneath it, the ice melts, but the temperature of the mixture is 

 never raised above that of so long as some of the ice remains 

 unmelted. The heat imparted changes ice at zero into water 

 at zero, but the thermometer indicates no increase of temperature. 

 Hence physicists say that heat has become latent, and that water 

 contains a certain quantity of latent heat beyond that of ice at 

 the same temperature. 



On the other hand, when we withdraw more heat from 

 the mixture of ice and water, the water gradually freezes : but 



