ME. HOPKINS ON THE THEOET OF THE MOTION OF GLACIEES. 
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mass would do. This was equivalent to saying that the mass was viscous because it 
moved in a particular manner, instead of asserting that the mass moved in that parti- 
cular manner because it was viscous. Now this kind of inversion of the direct enuncia- 
tion of the proposition is only admissible when there is no other physical cause, than the 
one assigned, to which it is conceivable that the observed phenomenon should be 
ascribed. Thus we may assert with perfect conviction, that gravity exists as a property 
of matter and acts according to a certain law, because the bodies of the solar system 
move as if such were the case ; but the conclusiveness of this inductive proof of the 
proposition — that “gravity is a property of matter” — rests entirely on our conviction 
that matter has no other property by which we could equally account for the phenomena 
of the celestial motions. And so with regard to glaciers. If viscosity were the only 
conceivable property of ice by which we could possibly account for the observed motion 
of glaciers, then would the observed phenomena of that motion perfectly convince us of 
the existence of the property in question. But here the two cases entirely diifer, 
inasmuch as there was no general conviction, nor even a decided probability at the time 
I allude to, that no physical property of ice could exist besides viscosity which might 
account for the observed phenomena of a glacier’s motion ; and at the present time it is 
proved that there is another property of ice by which those phenomena are perfectly 
accounted for, and the inductive proof of viscosity becomes altogether valueless. 
Moreover, in the case of universal gravitation, the inductive proof is the only possible 
one, whereas in glacial motion we are concerned with a property which, in whatever 
sense the definition of it may be regarded, must be as capable of being rendered patent 
by experiment in ice, if it exist, as in any other substance. 
The answer, then, that was given to the question, what is viscosity "? comprised no 
definition at all of that term. The viscous theory ignored the possibility of the mole- 
cular mobility of a glacial mass, united with the preservation of its continuity, being 
attributable to any other property than that which was designated as viscosity, but 
without giving any exact definition of the term. If it was meant to define by it the 
property which I have defined by the same term, the theory had a legitimate claim to 
be considered a jyhysical theory, because it assigned a determinate physical property as 
the cause of certain observed phenomena. In this sense, however, I conceive that it 
would now be admitted to be entirely disproved by Dr. Tyndall’s experiments, in which 
the ice exhibits so clearly the property of solidity, and the absence of all indication of 
plasticity. The hypothesis of viscosity, I imagine, could only have been adopted in the 
first instance from the apparent absence of any other property of ice which might 
account equally well for the molecular mobility of the glacial mass. 
4. But if the determinate property of viscosity, as I have defined it, be not recognized 
in ice, what, it will be asked, is really the idea which has been attached to the term 
plastic or viscous 1 The question, as I have already intimated, is difficult to answer. 
Perhaps the best way of doing so is to refer to the “Prefatory Note” to Principal 
Foebes’s ‘ Occasional Papers ’ (p. xvi). He there intimates that the expressions “ bruising 
