tinuous alteration of form is only produced by stresses exceeding a certain value, the substance is called a solid, however soft it may be. When the very smallest stress, if continued long enough, will cause a constantly increasing change of form, the body must be regarded as a viscous fluid, however hard it may be. Thus a tallow candle is much softer than a stick of sealing-wax; but if the candle and the stick of sealing-wax are laid horizontally between two supports, the sealing-wax will in a few weeks in summer bend with its own weight, while the candle remains straight. The candle is therefore a soft solid, and the sealing-wax a very viscous fluid.”[1]
Wax behaves in a similar manner to tallow. A wax figure can exist for a century without collapsing, if only the temperature does not attain the melting-point, whilst the same figure worked in sealing-wax would gradually flow away.
In nature there are all transitions between these two extremes of Maxwell, and even the examples given do not really form these extremes. Thus an infinitely small impulse is certainly not sufficient to cause deformation of sealing-wax, but the limit is so low that it begins to flow “of itself,” that is, under its own weight. In any case composite bodies such as form the silicate mantle of the earth must show characters of both kinds. If, therefore, neither sima nor sial can be compared with one of Maxwell’s prototypes, yet in my opinion a comparison with these throws much light on the processes of displacement which are so difficult for our imagination to grasp. A considerable difference is shown between both these substances, which we cannot explain better than by comparing the sima with sealing-wax and the sial with wax or tallow. Sima is the harder material (basalt is the best paving stone!),
- ↑ J. C. Maxwell, Theory of Heat, 2nd edition, p. 274, 1872.
