744 
ME. HOPKINS ON THE THEOET OF THE MOTION OF GLACIEES. 
fundamental physical principle on which his Viscous Theory rested, and without the expe- 
rimental verification which it demanded, and, moreover, without the guidance of those 
more refined and exact mechanical researches without which I am sure we can see our 
way but dimly through the more complicated of those problems which glacial theor\^ 
presents to us. Hence it is that I have been led to dissent from most of his speculative 
views. That dissent has been unreservedly expressed in this paper, and therefore it is 
that, in concluding it, I would bear testimony to what I consider the great and legiti- 
mate claims which the author of the Viscous Theory (however we may differ from the 
theory itself) has established to be regarded as one of the leading promoters of glacial 
science. 
After the greater part of this paper was written, a very interesting memoir by the 
Master of the Mint, “ On Liquid Diffusion applied to Analysis,” was brought under my 
notice. He there speaks of the “ colloidal condition of matter ” (of which gelatine seems 
to afford the type) as opposed to the “ crystalloidal condition.” He remarks (Philoso- 
phical Transactions, Vol. 151, Part I. 1861), “ Ice itself presents colloidal properties at 
or near its melting-point, paradoxical though the statement may appear.” Again, “ Ice, 
although exhibiting none of the viscous softness of pitch, has the elasticity and tendency 
to rend seen in colloids.” “ It further appears to be of the class of adhesive colloids. 
The redintegration (regelation of Faraday) of masses of melting ice when placed in 
contact, has much of a colloid character. A colloidal view of the plasticity of ice 
demonstrated in the glacier movements will readily develope itself” 
These passages were written without any direct reference to glacial questions ; but it 
occurred to me that a glacialist might possibly put constructions on them unfavom'able 
to the Hews I have expressed respecting the solidity of ice, and the absence in that sub- 
stance of any property which could, according to my own definition of the term, be called 
plastic. Consequently I wrote to the author requesting him to give me some further 
elucidation of his views on one or two points bearing on my own definitions and explana- 
tions. I proposed to him the following questions, to which he kindly gave me the 
subjoined clear and explicit replies. 
(1) “ Is the tendency to a colloidal character in ice, as opposed to a vitreous, crystal- 
line brittle structure, sufficient to interfere materially with the restitution described *, by 
giving the ice a greater degree of plasticity!” Mr. Graham’s answer was, “I believe 
not. A colloid, on the contrary, is often as nearly perfectly elastic as possible. Take 
the gluey material used to form the roller by which ink is applied in book-printing, as 
an illustration of the elasticity of a colloid, with the entire absence (apparently) of true 
plasticity or viscosity.” 
(2) “ Will the colloidal state of ice at temperature = zero (C.) materially modify the 
modus operandi (assuming the ice to be solid), rendering it approximate to the process 
which would take place supposing the mass to be plastic % ” Mr. Graham replies, 
* See art. 1. 
