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rigidity and permanency of form, characteristic of a solid. 
There is no passage through, a viscous condition intermediate 
between the liquid and solid states, as we find, for example, 
in wax, which softens before it melts. But with wax, as 
with most substances, there is no increase of volume takes 
place, at any point, as the cooling goes on. In this respect 
water is the most remarkable exception to a nearly uni- 
versal law, but the contraction is not uniform in its 
rate. Wax melts at 64°C. ; at the moment of solidifica- 
tion there is a sudden, small contraction, amounting to J per 
cent, (the expansion of water, in freezing, amounts to no 
less than 9 per cent.), and the melted wax then contracts, 
at first rapidly, but afterwards with decreasing rapidity. The 
hrittleness of ice is familiar to every one. A slight pressure 
with the point of a pin suffices to split a small lump, and it 
refuses to bend between the fingers. It is very unlike such a 
viscous substance as warm sealing-wax, a stick of which, 
long before it melts, bends easily even under its own weight. 
Yet we shall, I think, arrive at the conclusion, that in conse- 
quence of its peculiar properties, ice in such masses as glaciers 
and ice-sheets, behaves exactly as a viscous fluid would do. 
It may be well, before we proceed further, to state definitely 
in what exact sense several terms we shall have to use are to 
be understood. A rod of any substance, say of glass, firmly 
held at one end and weighted at the other, experiences, in 
consequence of this stress, a strain, shown by its bending. 
The force, or S3^stem of forces, which alters the form of a body 
is called stress — the change in form itself, strain. A body 
which, when subjected to stress experiences no strain, would 
be a perfectly rigid body. IS'one such exist. 
If the stress, after acting for a time, be removed, and 
the body returns to its original form, it is elastic, but if 
the form of the body be permanently altered, the body is 
soft or plastic. If the change of form of the body increase 
