PROFESSOR AT HEIDELBERG 229 



After the lecture du Bois writes to him on June 8, 1866 : 



'You see that I am somewhat rabid as usual, when I cannot 

 hammer out my own work, and see others shaking one fine thing 

 after another out of their lap. Our good Tyndall will be not 

 a little astonished to find you a master of glacier problems also.' 



In the lecture above referred to, which was published in the 

 Philosophical Magazine for the following year, with the title 'On 

 the Regelation of Ice', Helmholtz confirmed James Thomson's 

 explanation of the phenomenon of the regelation of ice at zero, 

 when two pieces of ice if pressed together freeze again and form 

 one mass. He proves that the freezing-point is lowered with 

 increased pressure, and points out, as against Faraday, that time 

 is an essential factor in this phenomenon; he shows by a number 

 of experiments that with strong pressure two pieces of ice can 

 be united into one block by the freezing water at their surface 

 of contact, while under weaker pressure it is necessary to wait 

 longer, and the parts are correspondingly easier to separate 

 again. He finds the plasticity of ice most marked in that which 

 has been welded together by great pressure from snow, while the 

 regular, crystalline ice can indeed be united by regelation, but 

 only forms a mass of irregular pieces. 



By applying these observations to glaciers, Helmholtz was 

 able to explain the well-known and never properly interpreted 

 phenomenon of the flow of ice in glaciers as a viscous mass. 

 The ice mass of a glacier is everywhere permeated with runnels 

 of water, so that its internal temperature is always at freezing- 

 point, seeing that the water would freeze if the temperature 

 were lower, and the ice would melt if it were higher. But a 

 mixture of ice and water grows colder and colder in proportion 

 to the pressure exerted upon it ; as no heat is withdrawn, the 

 free heat must become latent, and the ice in the mixture melts. 

 The pressure exerted by the glacier mass, which forces the 

 water out of the cracks, will, in Helmholtz's opinion, cause the 

 compressed ice (since its melting-point is lowered by pressure, 

 while the freezing-point of the non-compressed water is not 

 lowered) to give ice which is colder than o, in contact with 

 water at o. There will accordingly be constant congelation of 

 the compressed ice-water round it, with the formation of new 

 ice, while a portion of what is compressed melts simultaneously, 

 and the ice itself moves as a viscous fluid mass. 



