With regard to the power which sets bounds to the expansion of the fluid acting as heat, it is natural to think that it can be no other than the same fluid acting in a contrary direction, or from a circumference towards a centre; and thus we shall always find that the same fluid, by limiting its own operations, may produce those phenomena which have been hitherto deemed so difficult of explanation. In what manner this limitation is in all cases effected, or indeed in any case, we cannot pretend to explain. It is sufficient to observe, that wherever there is a perpetual _efflux_ of any thing, there must be also a perpetual _influx_ at the same time, and in proportion to the one the other will be. These two are directly contrary to one another, and, as we suppose the fluid to be universal, it is evident, that if any part of it be put in motion in a particular direction, the rest will press towards that part where the motion is, in order to keep up the equilibrium. Hence we may easily account for the heat produced by percussion or by friction. By hammering a piece of iron, as Dr. Black justly observes, the fluid is forced out from between the parts of the metal. The emission of this fluid in all directions is heat itself; and no sooner is one quantity thrown out than another supplies its place with great rapidity, and so on, until the pressure of the rest in some way or other counteracts the emission of any more, and the heat ceases. Just so with friction. The heat produced by it is always in proportion to the pressure employed. By this pressure the parts of the two substances are forced into such close contact, that an agitation and emission of the fluid pervading their substance takes place. This agitation, as we have already noticed, is heat itself, and, as long as the friction is continued, more and more heat will be produced, without any limitation, as Count Rumford has observed. Some bodies have a greater disposition than others to emit this subtile fluid; and these we say are naturally of a warmer temperature than others. The _temperature_ is nothing else than the efflux of the fluid from them, continually kept up by the action of the surrounding fluid. By mixture with different substances the temperatures of various bodies may be changed; by some the influx, and by others the efflux, may be augmented. In the former case we say the body becomes colder, in the latter hotter, than before; and in not a few cases the agitation of the fluid becomes so great that the matter actually takes fire. In all these cases, however, we can discover nothing more than the bare fact, that so and so is the case. We know that the bodies do grow hot by the convergence of the etherial fluid towards them, and its emission from them; but why it should converge or diverge we know not.