682 
ME. HOPKINS ON THE THEOET OF THE MOTION OF GLACIEES. 
and re-attachment,” and “ incipient fissures re-united by time and cohesion,” used by 
him in 1846, are to be regarded as having the same meaning as the expression “fracture 
and regelation,” first introduced into the subject in 1857. No-vv there is no ambiguity 
whatever in this latter expression. “ Fracture” means the breaking and splitting of the 
ice regarded as a brittle and crystalline solid, and could never be intended to have the 
slightest reference to viscosity. In fact the expression is altogether inapplicable to any 
body which can be called viscous, without what I should regard as a violation of scien- 
tific language. Still this, it may be said, may be only a want of strict accuracy of expres- 
sion, rather than of accuracy of conception. But if a notion of cracking and breaking, so 
foreign to any idea of plasticity, should be admitted, it could not be said that a glacier 
moved as it is observed to move because it was plastic, but merely that it moved as if 
it were plastic. The true inference from the motion would have been that glacial ice 
possessed not necessarily real plasticity, a definite property of bodies, but a quasi-elasti- 
city, which expresses no determinate property at all, but may consist with many different 
properties. It merely expresses, in fact, the power of the component elements of the 
mass of changing to a certain extent their relative positions. But this is not the pecu- 
liar property of ice ; it is common, indeed, to all bodies exposed to disruptive forces 
which, as in the case of ice, the cohesive power is unable to withstand. The mass of 
any other substance, as well as that of a glacier, will then be broken into fragments 
sufiiciently small to allow it to follow the impulses of the external forces acting on it. 
To say, therefore, that a glacier moves as if it were plastic is not to assign to ice any 
property peculiar to itself, and therefore does not properly constitute a physical theory 
of glacial motion at all. 
5. But if we should pass over the difference between true plasticity and that which, 
as we have pointed out, is merely apparent, there would still remain the great difficulty 
which was only removed by the experiments of Mr. Faeadat and Dr. Tyndall. Every 
one who believed ice to be a solid body, believed as a matter of necessity that a glacier 
must, on account of the external conditions to which it is subjected, be excessively broken 
and dislocated in the course of its motion. I was myself one of those who fell into the 
error of attributing too much infiuence to the larger and more visible disruptions of the 
mass ; but the great difficulty was in the perfect subsequent reunion of portions which 
had thus been separated, whether by larger or smaller dislocations. And here it 'will 
necessarily be asked whether, in the expression above quoted, “ re-attachment ” and the 
'^'■re-union by time and cohesion” of separated portions when again brought into contact, 
really mean the same thing as regelation % It can only be answered, I thmk, by saying 
that, whatever might be the intended meaning of those expressions, they failed to convey 
to the minds of others the most remote idea of regelation as a property of ice at a 
particular temperature. No better proof can be given of this than the general con- 
viction which appeared to flash across the mind of every glacialist when he first heard 
of Dr. Tyndall’s experiment, that the recognition of the property of instantaneous 
regelation was a well-marked and important discovery, which had at once completely 
