Page:Elementary Text-book of Physics (Anthony, 1897).djvu/138

124 Upon these facts we may base a distinction between solids and fluids: a solid requires the stress acting on it to exceed a certain limit before any permanent set occurs, and it makes no difference how long the stress acts, provided it lies within the limit. A fluid, on the contrary, may be permanently deformed by the slightest shearing stress, provided time enough be allowed for the movement to take place. The fundamental difference lies in the fact that fluids have no rigidity and offer no resistance to shearing stress other than that due to internal friction or viscosity.

A solid, if it be deformed by a slight stress, is soft; if only by a great stress, is hard or rigid. A fluid, if deformed quickly by any stress, is mobile; if slowly, is viscous.

It must not be understood, however, that the behavior of elastic solids under stress is entirely independent of time. If, for example, a steel wire be stretched by a weight which is nearly, but not quite, sufficient to produce an immediate set, it is found that, after some time has elapsed, the wire acquires a permanent set. If, on the other hand, a weight be put upon the wire somewhat less than is required to break it, by allowing intervals of time to elapse between the successive additions of small weights, the total weight supported by the wire may be raised considerably above the breaking-weight. If the weight stretching the wire be removed, the return to its original form is not immediate, but gradual. If the wire carrying the weight be twisted, and the weight set oscillating by the torsion of the wire, it is found that the oscillations die away faster than can be explained by any imperfections in the elasticity of the wire.

These and similar phenomena are manifestly dependent upon peculiarities of molecular arrangement and motion. The last two are exhibitions of the so-called viscosity of solids. The molecules of solids, just as those of liquids, move among themselves, but with a certain amount of frictional resistance. This resistance causes the external work done by the body to be diminished, and the internal work done among the molecules becomes transformed into heat.