398 Prof. Tyndall on Ice and Glaciers. 



Professor Helmholtz then explains to his audience what is 

 meant by latent heat, and points out that, through the circula- 

 tion of water in the fissures and capillaries of a glacier, its inte- 

 rior temperature must remain constantly at the freezing-point 

 of water. 



" But," he continues, " the temperature of the freezing-point 

 of water can be altered by pressure. This was first deduced by 

 James Thomson, and almost simultaneously by Clausius, from 

 the mechanical theory of heat ; and by the same deductions even 

 the magnitude of the change may be predicted. For the pres- 

 sure of every additional atmosphere, the freezing-point sinks 

 o, 0075 C. The brother of the gentleman first named, William 

 Thomson, the celebrated Glasgow physicist, verified experimen- 

 tally the theoretic deduction by compressing a mixture of ice and 

 water in a suitable vessel. The mixture became colder and colder 

 as the pressure was augmented, and by the exact amount which 

 the mechanical theory of heat required. 



" If, then, by pressure a mixture of ice and water can be ren- 

 dered colder without the actual abstraction of heat, this can only 

 occur by the liquefaction of the ice and the rendering of heat 

 latent. And this is the reason why pressure can alter the point 

 of congelation * * * *. 



" In the experiment of William Thomson just referred to, ice 

 and water were enclosed in a solid vessel from which nothing 

 could escape. The case is somewhat different when, as in the 

 case of a glacier, the water of the compressed ice can escape 

 through fissures. In this case the ice is compressed, but not the 

 water which escapes. The pressed ice will become colder by a 

 quantity corresponding to the lowering of its freezing-point by 

 the pressure. But the freezing-point of the uncompressed water 

 is not lowered. Here, then, we have ice colder than 0° C. in 

 contact with water at 0° C. The consequence is, that round the 

 place of pressure the water will freeze and form new ice, while, 

 on the other hand, a portion of the compressed ice continues to 

 be melted (wahrend dafiir ein Theil des gepressten Eises fort- 

 schmilzt). 



" This occurs, for instance, when two pieces of ice are simply 

 pressed together. By the water which freezes at the points of 

 contact they are firmly united to a continuous mass. When the 

 pressure is considerable and the chilling consequently great, the 

 union occurs quickly, but it may also be effected by a very slight 

 pressure if sufficient time be afforded. Faraday, who discovered 

 this phenomenon, named it the revelation of ice*. Its explana- 

 tion has given rise to considerable controversy : I have laid that 



* I have already corrected this slight inadvertence. 



