842 HELMHOLTZ 



possible that ice, which is the most brittle and fragile of 

 substances, can flow in the glacier like a viscous mass ; and 

 you may perhaps be disposed to regard this as one of the 

 wildest and most improbable statements that have ever been 

 made by philosophers. I will at once admit that philoso- 

 phers themselves were not a little perplexed by these results 

 of their investigations. 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 cor- 

 rectly remarked, this constituted an essential difference be- 

 tween 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 he'at. 

 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 with- 

 out melting. Ice and water can exist in each other's pres- 

 ence 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 o 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 in- 

 crease 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. 



