Prof. Tyndall on Ice and Glaciers. 405 



Forbes. This theory, for it truly is a theory, basing itself on 

 facts as numerous as they are well observed, enunciates the prin- 

 ciple that ice possesses the characteristic properties which belong 

 to plastic bodies. Although he did not directly prove it, to Pro- 

 fessor Forbes belongs not the less the great merit of insisting on 

 the plasticity of ice, before Faraday, in discovering the pheno- 

 menon of regulation, enabled Tyndall to prove that the plas- 

 ticity was real, at least partially. 



" The experiment of Faraday is classical in connexion with our 

 subject. It consists, as you know, in this, that if two morsels 

 of ice be brought into contact in water, which may be even warm, 

 they freeze together. Tyndall saw very quickly the application 

 of Faraday's experiment to the theory of glaciers; he com- 

 prehended that, since pieces of ice could thus solder themselves 

 together, the substance might be broken, placed in a mould, 

 compressed, and thus compelled to take the form of the cavity 

 which contained it. A wooden mould, for example, embraces a 

 spherical cavity ; placing in it fragments of ice and squeezing 

 them, we obtain an ice sphere ; placing this sphere in a second 

 mould with a lenticular cavity and pressing it, we transform the 

 sphere into a lens. In this way we can impart any form what- 

 ever to ice. 



" Such is the discovery of Tyndall, which may well be thus 

 named, particularly in view of its consequences. For all these 

 moulds magnified become the borders of the valley in which a 

 glacier flows. Here the action of the hydraulic press which has 

 served for the experiments of the laboratory is replaced by the 

 weight of the masses of snow and ice collected on the summits, 

 and exerting their pressure on the ice which descends into the 

 valley. Supposing, for example, between the spherical mould and 

 the lenticular one, a graduated series of other moulds to exist, each 

 of which differs very little from the one which precedes and from 

 that which follows it, and that a mass of ice could be made to 

 pass through all these moulds in succession, the phenomenon 

 would then become continuous. Instead of rudely breaking, the 

 ice would be compelled to change by insensible degrees from the 

 spherical to the lenticular form. It would thus exhibit a plas- 

 ticity which might be compared to that of soft wax. But ice 

 is only plastic under pressure ; it is not plastic under tension : 

 and this is the important point which the vague theory of plas- 

 ticity was unable to explain. While a viscous body, like bitumen 

 or honey, may be drawn out in filaments by tension, ice, far from 

 stretching in this way, breaks like glass under this action. These 

 points well established by Tyndall, it became easy for him to 

 explain the mechanism of glaciers, and by the aid of an English 

 geometer, Mr. William Hopkins, to show how the direction of 



