Rev. J. F. Blake — On the Motion of Glaciers. 495 



physics, with something much larger, equivalent in the popular mind 

 with " a very little bit," and having no connexion with the molecules 

 that rearrange themselves in freezing. 



A true " molecule " cannot be " crystalline," because you cannot 

 arrange one thing except with respect to others. You might as well 

 speak of a single soldier drawn up in a hollow square, and when 

 the molecule " melts," of the soldier being disbanded. 



When, therefore, Mr. Croll writes in this way. Prof. Tyndall and 

 he part company, and it is difficult to know what sense to put upon 

 what follows ; with that one phrase of his the whole edifice falls. 



Using, however, the word molecule in its proper sense, it may be 

 worth while to follow a little what might possibly happen in the 

 case supposed. Writing what Mr. Croll would express, in strict 

 language, he may be supposed to say that, when, by the action of 

 heat, several molecules adjacent to each other lose their polarity and 

 the crystalline substance melts, some of these molecules will fit better 

 into the spaces left between those still forming part of the crystalline 

 substance, and better also among themselves, so that on the whole 

 they occupy less room than before.^ This, of course, is true. He 

 then says that these molecules, on resolidifying, will not do so so as 

 to fit the cavity which they occupied when in a fluid state. Taking 

 this assertion as I have modified it, so as to represent what alone 

 can be Mr. Croll's meaning, I think it cannot be upheld ; but of 

 that anon. I would examine his reasons. He says, " The liquid 

 molecule in solidifying assumes the crystalline form, and of course 

 there will be a definite proportion between the length, breadth, and 

 thickness of the crystal ; consequently it will always happen that 

 the interstice in which it solidifies will be too narrow to contain 

 it." Here, I must say, the confusion seems complete ; for whatever 

 definition we give of a molecule or an interstice, we are equally 

 landed in difficulties. Mr. Croll's idea of a molecule, from this 

 sentence, would seem to be a small portion of the substance which, 

 when melted, may take any shape, but which, when solid, must take 

 up a definite crystalline shape. Js this small portion of the sub- 

 stance divisible or indivisible under the ordinary forces of nature ? 

 If divisible, why should it not flow into several interstices and be 

 squeezed into more still in the act of solidifying ?— and Mr. Croll says 

 it flows into the " interstices " (plural) — where it might make several 

 smaller crystals than the original, according as there was room. If 

 indivisible — and Mr. Croll further down speaks of "the interstice" — 

 the main mass would remain in the original cavity, and by the 

 hypothesis of indivisibility could only crystallize by coming back 

 together, or by the main mass coming after the minute portion that 

 had got entangled in an interstice. Mr. Croll seems to think the 

 former could not be the case, as he says that in crystallizing "the 

 molecule will press the two adjoining molecules aside in order to 

 make sufficient room for itself between them, and this it will do, no 

 matter what amount of space it may possess in all other directions. 



^ This phraseology would have to be altered if we consider heat to consist of the 

 motion of molecules, but I have kept as close to Mr. Croll's language as possible. 



