This doctrine also explains the effect of freezing mixtures, as that of salt and snow. These solid substances, on being mixed, become fluid, and that fluid absorbing much heat, deprives all the neighbouring bodies of part of what they had. But if the temperature at which the mixture is made be as low as that to which this mixture would have brought it, it has no effect, and in a lower temperature this new fluid would become solid; for that mixture has only a certain determinate capacity for heat, and if the neighbouring bodies have less heat, they will take from it. It has been observed, that the comparative heat of bodies containing phlogiston is increased by calcination or combustion; so that the calx of iron has a greater capacity for heat, and therefore contains more latent heat, than the metal. In general it is not found, that the same substances have their capacity for receiving heat increased by an increase of temperature; but this is said to be the case with a mixture of ardent spirit and water, and also that of spirit of vitriol and water. Since all substances contain a greater or less quantity of heat, and in consequence of being deprived of it become colder and colder, it is a question of some curiosity to determine the extent to which this can go, or at what degree in the scale of a thermometer any substance would be absolutely cold, or deprived of all heat; and an attempt has been made to solve this problem in the following manner. Comparing the capacity of water with that of ice, by means of a third substance, viz. mercury, it has been found, that if that of ice be 9 deg., that of water is 10 deg.; so that water in becoming ice gives out one tenth part of its whole quantity of heat. But it has been shown, that ice in becoming water absorbs 146 degrees of heat. This, therefore, being one tenth part of the whole heat of water, it must have contained 1460 degrees; so that taking 32 degrees, which is the freezing point, from that number, the point of absolute cold will be 1426 below 0 of Fahrenheit's scale. By a computation, made by means of the heat of inflammable and dephlogisticated air, at the temperature of 50, Dr. Crawford finds, that it contains nearly 1550 degrees of heat; so that the point of absolute cold will be 1500 below 0. But more experiments are wanted to solve this curious problem to entire satisfaction. LECTURE XXXI. _Of Animal Heat._ Since all animals, and especially those that have red blood, are much hotter than the medium in which they live, the source of this heat has become the subject of much investigation; and as the most probable theory is that of Dr. Crawford, I shall give a short detail of the reasons on which it is founded.