614 THE LIFE OF JAMES D. FORBES. [CHAP. 



an equilibrium of heat, and therefore no appreciable 

 internal fusion can occur. 



' 4. According to my view ice is only hard and crystal- 

 line at a temperature of 30 or even lower [Occas. Papers, 

 p. 225]. At 32 it is a clammy, plastic solid, not unlike 

 Cheshire cheese ; and it behaves very like it under the 

 press or the knife. Such a substance cannot be bruised 

 and the bruised surfaces brought into contact with 

 moderate pressure, without reattachment, as a simple 

 result of "time and cohesion" [Occas. Papers, xxi., 20 ij 

 without appealing to any other properties of ice or effects 

 of pressure. Mr. Graham's opinion that ice is a "colloid," 

 entertained on quite independent grounds, confirms this 

 view. 



'5. I consider ice at 32 to have a slight, but perfectly 

 sensible, genuine molecular plasticity. When the urgency 

 of the forces to which it is subjected causes this limit to 

 be overpassed, the pasty solid is subject to a general 

 bruise, producing innumerable internal fissures with 

 finite displacements, and subsequent restoration of con- 

 tinuity under pressure. Hence the blue bands. When 

 the urgency of gravity is still greater, of course the 

 mass acts as a common solid, and crevasses open. [See 

 Occas. Papers, p. 161, &c., and many other places.] 



' It must have been remarked that I have never 

 attempted to reply to Mr. Hopkins' mathematical demon- 

 strations about glacier motions and the internal forces by 

 which they are controlled : and for this very good reason, 

 that I never felt myself mathematically strong enough to 

 replace his alleged demonstrations by others which I 

 could regard as any true representations of the problem 

 presented by Nature. 



* From general physical considerations, and from an 

 experimental study of plastic bodies, I was well aware 

 that Mr. Hopkins' mode of treatment was applicable 

 only to a body possessing properties, or placed under 

 conditions, which I was perfectly certain that a glacier 

 had not or was not placed in. I regarded his solutions 

 therefore (I refer to his early papers in the 



