IV 



For the sake of clearness, ifc may be well to leave here for a moment 

 the chronological order of James Thomson's life, and to explain 

 briefly the subsequent development of the ideas first disclosed in 

 this paper of 1849. 



Forbes had discovered, by observations and experiments on the 

 Swiss glaciers, the property of plasticity in ice. The fact of plasticity 

 in ice was at first doubted ; but it was afterwards admitted, and 

 various explanations were offered of this property, so remarkable in 

 a brittle and, above all crystalline, substance. 



In this connection, Faraday called attention to the freezing together 

 of two pieces of ice placed together in water; and from this arose 

 a partial explanation, by Tyndall, under the designation of " Fracture 

 and Regulation. " But the theory, and even the not logical juxta- 

 position of the two words, did not satisfy James Thomson. There 

 was nothing to show why or how reunion (or " regelation ") should 

 take place after fracture. He saw, however, that an extension of his 

 own previous principle of lowering of the freezing point by pressure 

 allowed him to apply it to the effect of distorting stress on solid ice> 

 and would give a perfect explanation of all Faraday's observations 

 and experiments on the union and growth of the connecting link 

 between two pieces of ice under water, pressed together by any force, 

 however small. 



By this extended thermodynamic principle he also accounted for 

 the yielding of a mass of ice crystals (diy snow, for instance) at 

 temperatures lower than the ordinary freezing point. He demonstrated 

 that the mutual pressures must melt the ice at, and close around, 

 the points of contact ; and that, when there is relief from the internal 

 stress by this melting, the low temperature of the main solid mass, 

 and the extra cold due to the latent heat required for liquefac- 

 tion of the yielding portions, cause the melted matter to re-freeze 

 in the places to which it has escaped in order to relieve itself from 

 strain. Thus a complete explanation, based on a demonstrated physical 

 principle, was offered of the phenomenon. 



Thomson's explanation did not, certainly at first, commend itself 

 thoroughly to Faraday. A very interesting correspondence between 

 them ensued ; and Faraday made a number of beautiful and interest- 

 ing experiments, with the object of showing that the placing of two 

 pieces of ice on opposite sides of a film of water (between them) 

 would give rise to the conversion of the film of water into ice, and 

 cause the union of the two pieces of ice, the principle being that of 

 the starting of crystallisation in a supersaturated solution by mean,s 

 of a crystal of the solid. James Thomson, however, showed that, in 

 the experiments adduced by Faraday, pressure between the ice blocks 

 was not absent. For example, in an experiment in which two pieces 

 of ice, with a hole through each, were mounted on a horizontal rod of 



