ME. HOPKINS ON THE THEOET OF THE MOTION OF GLACIEES. 
746 
“ Quite the contrary. The only influence to be looked for from the colloidal state would 
be greater elasticity — that is, the matter would be likely to yield more before rending or 
breaking, supposing it to be colloidal, than if it were crystalline. Your mechanical con- 
ception of the modus O'perandi in the fracture of a solid (not plastic) applies equally well 
to a crystal or colloid. I go entirely with you up to that point.” 
Mr. Geaham further remarks, “ Colloidality is invoked chiefly with the view of account- 
ing for the ready reunion, the redintegration as I have called it, of fragments of ice 
brought into contact with each other. It is a very general (perhaps universal) character 
of colloids to adhere and reunite when two masses are pressed together. In fact all our 
adhesive substances, gum, glue, starch, &c., belong to the class of colloids. Even glass, 
which is a colloid of fusion, shows the adhesive character, two sheets of polished plate 
glass often adhering so thoroughly as to tear up each other’s surface when forcibly sepa- 
rated. Another colloid adhesive like ice, is fused phosphoric acid — ‘ glacial ’ phosphoric 
acid as it is called, in prescience, one might imagine, of this discussion ! No such adhe- 
sive property is ever found in crystalline surfaces, so far as I am aware. It is qua colloid 
that ice appears to be adhesive. The discovery of Faeaday’s, of the adhesive quality of 
ice, is the fundamental fact of the glacier discussion. The name ‘ regelation ’ applied 
to it may, however, be objected to, being quite speculative, and implying, as it appears 
to do, that two pieces of ice come to be cemented together by the freezing of a film of 
water between them, instead of simply adhering perfectly and uniting as two pieces of 
plate glass might do. The great fact, however, remains, and the name is but a trivial 
matter.” 
I also thought it right to inquire of Mr. Geaham the exact meaning he intended to 
attach to the word “ demonstrated ” in the last sentence of the above quotation from his 
memoir. A glacialist might possibly, I thought, if so disposed, interpret it as meaning 
“proved” or “fully established.” In his answer Mr. Geaham says, “For ‘demon- 
strated ’ I should have said ‘indicated’ as you suggest.” He also remarks, “You will, 
I am certain, do good service in the glacier discussion by the precision you introduce 
into the terminology of the subject*. I accept without reserve your deflnitions of plas- 
ticity, &c. In the passage you quote from my paper, I had spoken of ice as plastic in a 
more general sense, as plasticity is understood, I apprehend, by Professor FoEBEsf. A 
solid was looked upon as plastic of which the form can be remodelled anyhoiv — by 
fracture and reunion, as well as in consequence of viscosity.” 
These views of the Master of the Mint are so explicitly and clearly expressed that 
they need no comments on my part, beyond the expression of the high gratification I 
derive from the coincidence of the opinions stated in his letter with those which have 
been put forth in this memoir. 
* I had communicated my definitions to Mr. Geaham. 
t This is what I termed quasi-elasticity, or apparent plasticity . 
