This is a common property of all liquids. If they be exposed for a sufficient length of time to a sufficient degree of heat, they will always be converted into elastic fluids. These are usually distinguished from air and other permanent gases, which never are known to exist in the liquid form, by the term _vapour_, by which, therefore, must be understood an elastic fluid which at common temperatures exists in the liquid or solid state; by _steam_ is expressed the vapour of water; and by _gases_, those elastic fluids which like air are never known—at least, under ordinary circumstances—to exist in any other but the elastic form. (15.) When a liquid is caused, by the application of heat, to take the form of an elastic fluid, or is evaporated, besides acquiring the property of elasticity, it always undergoes a considerable change of bulk. The amount of this change is different with different liquids, and even with the same liquid it varies with the circumstances under which the change is produced. (16.) When water is evaporated under ordinary circumstances,—that is, when exposed to no other external pressure than that of the atmosphere,—it increases its volume about seventeen-hundred-fold. Thus a cubic inch of liquid [Pg030] water would form about seventeen hundred cubic inches of common steam. If, however, the water be confined by a greater pressure than that produced by the common atmosphere, then the increase of volume which takes place in its evaporation would be less in proportion. These important physical circumstances are now only indicated in a general way. As we proceed with our account of the invention and improvement of the steam engine, they will be developed more fully and accurately. (17.) After duly considering what has been just explained, no difficulty will be found in comprehending the principles on which the first rude attempts at the mechanical application of steam already stated depend. In the apparatus ascribed to _Hero_ of Alexandria, the elasticity of the vapour contained in the arms of the revolving ball causes that vapour to issue from the lateral orifices in the arms, such as that of G, _fig._ 1. As these orifices, however, are exposed to the common atmosphere pressing inwards with a force, the mean amount of which has been stated to be about fifteen pounds per square inch, it follows that the steam cannot escape from these orifices until its pressure or elasticity exceeds this amount, and that when it does, the force with which it will so escape will be the excess of its elasticity above that of the atmosphere; and it is the reaction produced by this difference of pressure, causing the arms to recoil, which will give motion to the machine.