494 Rev. J. F. Blake — On the Motion of Glaciers, 



seems to me, it may be useful to clear tlie way for a true one, wliicli 

 may perhaps be done without a personal knowledge of glaciers. 

 This paper is, therefore, rather critical of the theories of others, 

 particularly of that of Mr. Croll, than suggestive of any of my own. 



We have Mr. Croll's latest form of his theory extracted for us in 

 the August Number of the Geological Magazine, which we are to 

 regard as alone representing it, although in his book, " Climate and 

 Time," the original form in which it was criticized by Mr. Burns 

 was allowed to stand as well, and was understood by me in the same 

 sense as by the latter writer. But it is of very little consequence in 

 which form it is stated, and I will examine the ideas derivable from 

 the extract las-t given. 



Mr. Croll says, " Ice is evidently not absolutely solid throughout. 

 It is composed of crystalline particles, which, though in contact 

 with one another, are, however,, not packed together so as to occupy 

 the least possible space, and, even though they were, the particles 

 would not fit so closely together as to exclude interstices." In read- 

 ing this we must be careful to obtain a right idea of the size of these 

 interstices. The reference to Prof. Tyndall's explanation of the 

 cause of the expansion of water in freezing — a very probable ex- 

 planation — shows that they are interstices between single molecules, 

 and therefore too small to be even microscopic. Although this 

 structure of ice is said to be " evidently " the true one, it rests on no 

 basis but Prof. Tyndall's theory ; and therefore must be understood 

 as he would understand it. 



It might be thought by some that the question of interstices in 

 ice would be connected with its origin ; either as crystallizing directly 

 from water, or formed by compression of snow crystals, as in a 

 glacier ; but this would have reference to interstices of quite a 

 different magnitude, not at all molecular, though possibly micro- 

 scopic. No — the interstices in Prof. Tyndall's theory must exist in 

 the smallest possible crystal, in fact be in the essence of a crystal, 

 and therefore all ice must as far as they are concerned act as glacier- 

 ice. 



If Mr. Croll does not mean molecular interstices, then of course 

 the answer to his theory will be different ; but then also his re- 

 ference to Prof. Tyndall is in vain, and the supposed structure of ice 

 by no means " evidently " the true one. 



In the passage quoted by Mr. Croll, Prof. Tyndall clearly says, 

 "The molecules turn and rearrange themselves (the italics are mine), 

 demanding as they do so more space." It is the mutual relation of 

 one molecule to another that is altered in solidification, and not any 

 alteration in the shape or nature of a single molecule, which would 

 be absurd ; since by the definition of a molecule [which Mr. Croll 

 does not appear to know] any such change would alter the substance 

 from water to some other compound of its elements. 



When, therefore, Mr. Croll in his next paragraph says, " when a 

 crystalline molecule melts," he either uses the word " molecule " in 

 a sense different to that of Prof Tyndall and peculiar to himself, or 

 confuses the true molecule, as defined in any book on chemistry or 



