The expansion of a body into a greater bulk by its own elastic power. It differs from rarefaction: for though the effects of both are nearly, if not quite the same, yet the latter arises from the application of heat. It has been observed by modern philosophers, that bodies which have been compressed and are again set at liberty endeavour to dilate themselves with a force equal to that by which they are compressed; accordingly they are found to sustain a force and raise a weight equivalent to the force of compression. Bodies in the act of dilating by their own elasticity exert a greater force at the beginning than towards the end, as being at first more compressed; and the greater the compression is the greater is the elastic power and endeavour to dilate: hence the compressing power, the compression, and the elastic force, are necessarily equal, and may be taken the one for the other. The motion by which compressed bodies restore themselves is usually accelerated, though sometimes not.

When compressed air begins to restore itself, and dilate into a larger space, it is still compressed, consequently new impetus is continually impressed upon it by the dilatative cause; and the former remaining with this continual addition, the effect, namely, the velocity, must likewise evidently be increased. But it may also happen, that where the compression is only partial, the motion of dilatation will not only not increase, but be even retarded, as is the case when sponge, soft bread, gauze, and other similar bodies, are compressed.