158 



This supposition evidently contains two very distinct hypotheses. 

 The former, which has to do with ice and water present together, I 

 certainly do regard as an assumption, unsupported hy any of the 

 phenomena which Mr. Faraday has adduced. The other, which has 

 to do with a particle of ice in the middle of continuous ice, and 

 which assumes that it will not so readily change to water, as another 

 particle of ice in contact with water, I think is to be accepted as pro- 

 bably true. I think the general bearing of all the phenomena he has 

 adduced is to show that the particles of a substance when existing all 

 in one state only, and in continuous contact with one another, or in 

 contact only under special circumstances with other substances, ex- 

 perience a difficulty of making a beginning of their change of state, 

 whether from liquid to solid, or from liquid to gaseous, or probably 

 also from solid to liquid : but I do not think anything has been ad- 

 duced showing a like difficulty as to their undergoing a change of 

 state, when the substance is present in the two states already, or 

 when a beginning of the change has already been made. I think 

 that when water and ice are present together, their freedom to 

 change their state on the slightest addition or abstraction of heat, or 

 the slightest change of pressure, is perfect. I therefore cannot 

 admit the validity of Mr. Faraday's mode of accounting for the 

 phenomena of regelation. 



Thus the fact of regelation which Prof. Tyndall has taken as the 

 basis of his theory for explaining the plasticity of ice, does in my 

 opinion as much require explanation as does the plasticity of ice which 

 it is applied to explain. The two observed phenomena, namely the 

 tendency of the separate pieces of ice to unite when in contact, and 

 the plasticity of ice, are indeed, as I believe, cognate results of a com- 

 mon cause. They do not explain one another. They both require 

 explanation ; and that explanation, I consider, is the same for both, 

 and is given by the theory I have myself offered . 



I now proceed to discuss the experiment by Prof. Forbes, already 

 referred to as having been adduced in opposition to my theory. He 

 states that mere contact without pressure is sufficient to produce the 

 union of two pieces of moist ice * ; and then states, as follows, his 

 experiment by which he supposes that this is proved : " Two slabs 



* " On some Properties of Ice near its Melting-Point," by Prof. Forbes, Pro- 

 ceedings Royal Soc. Edin., April 1858. 



