Prof. Tyndall on Ice and Glaciers. 897 



subjected it to the test of experiment, and the conclusions which 

 he draws from his researches are substantially the same as mine. 



Thus, then, as regards the incapacity of ice to stretch in obe- 

 dience to tension, and its capacity to be moulded to any extent 

 by pressure — as regards the essential difference between a glacier, 

 and a stream of lava, honey, or tar — as regards the sufficiency of 

 pressure and regelation to account for the formation of glaciers, 

 and of fracture and regelation to account for their motion — as 

 regards, finally, the insufficiency of the theory which refers the 

 motion to liquefaction by pressure, and refreezing, the views of 

 Professor Hemholtz and myself appear to be identical. 



But the case is different with regard to the cause of regela- 

 tion itself. Here Professor Helmholtz, like M. Jamin*, accepts 

 the clear and definite explanation of Professor James Thomson 

 as the most satisfactory that has been advanced; and he sup- 

 ports this view by an experiment so beautiful that it cannot fail 

 to give pleasure even to those against whose opinions it is ad- 

 duced. But before passing to the experiment, which is described 

 in the Appendix to the lecture, it will be well to give in the 

 words of Professor Helmholtz the views which he expresses in 

 the body of his discourse. 



" You will now ask with surprise," he says, " how it is that 

 ice, the most fragile and brittle of all known solid substances, 

 can flow in a glacier like a viscous mass ; and you may perhaps 

 be inclined to regard this as one of the most unnatural and 

 paradoxical assertions that ever was made by a natural philo- 

 sopher. I will at once admit that the inquirers themselves were 

 in no small degree perplexed by the results of their investiga- 

 tions. But the facts were there, and could not be dissipated by 

 denial. How this kind of motion on the part of ice was possible 

 remained long an enigma — the more so as the known brittleness 

 of ice also manifested itself in glaciers by the formation of nume- 

 rous fissures. This, as Tyndall rightly maintained, constituted 

 an essential difference between the ice-stream, and a stream of 

 lava, tar, honey, or mud. 



" The solution of this wonderful enigma was found — as is often 

 the case in natural science — in an apparently remote investiga- 

 tion on the nature of heat, which forms one of the most important 

 conquests of modern physics, and which is known under the 

 name of the mechanical theory of heat. Among a great number 

 of deductions as to the relations of the most diverse natural 

 forces to each other, the principles of the mechanical theory of 

 heat enable us to draw certain conclusions regarding the de- 

 pendence of the freezing-point of water on the pressure to which 

 the ice and water are subjected." 



* Traite de Physique, vol. ii. p. 105. 



