Again, let us suppose that the etherial fluid enters the substance of any body capable of being dilated to a great degree, it is equally plain that the action of the fluid must for some time be directed only upon the internal parts, and consequently will be imperceptible on the outside. This then is called _latent_ heat; and where the pressure on the outside balances that on the inside no heat will be perceptible to the touch. But by whatever means this balance is broken, heat will instantly be perceptible; and experiments show that the balance may be broken either by an increase of cold or heat. Thus, in the case of water, the internal pressure remains equal to the external, until the fluid is cooled to a few degrees below 32. The balance is then broken, and the internal action prevails; a quantity of what is called _sensible_ heat escapes, and the water is converted into ice. Again, at the temperature of 32, little or none of the water evaporates; but by the addition of heat, by which the internal action of the subtile fluid we speak of becomes greater than the external, the water is converted into vapour; and it is remarkable that the same effect takes place on greatly augmenting the degree of cold; for the evaporation from ice, even in frosty weather, is found to be very considerable. On the whole, from innumerable experiments it appears, that there exists in nature a certain invisible fluid, by the action of which, when diverging from a centre, heat is produced in the central point. By a certain other power this diverging force is limited, so that in some cases it is not perceptible beyond the surface of the body in which it acts, and then it is called _latent_ heat. In other cases it is perceptible in a certain degree, and the degree in which it is perceptible hath been called the _temperature_ or _sensible heat_ of the body. On mixing different substances together it is found, that very often the proportions between the external and internal actions are varied. This has been already observed, when giving an account of Dr. Black’s discovery of _latent_ heat, viz. that when snow and warm water are mixed together the temperature of the mixture differs very considerably from the arithmetical mean between the temperatures of the two substances employed. Dr. Crawford prosecuted the experiment further, and found that there were few substances which, on being mixed, did not shew a temperature different from that of the arithmetical mean between the temperatures of the two originally employed. This difference he unfortunately used as the foundation of a rule for determining the _capacities_ of different substances for containing heat, and upon this erroneous principle has raised a superstructure, which upon no occasion can be of service to science, but must always produce obscurity and confusion wherever it is introduced.