6. We must now consider Dr. Girtanner’s account of the origin of animal heat, which is, that, “during the distribution of the oxygen through the system, the heat which was united with this oxygen escapes; hence the animal heat;” and, “that the great _capacity_ of the arterial blood for heat is owing to the oxygen with which it is united in the lungs.”--This leads us to consider in a more particular manner the doctrine of heat, a subject hitherto much less investigated than the importance of the subject requires. What little we do know of this matter seems to be almost entirely owing to Dr. Black, who hath discovered some very remarkable phenomena unknown to former philosophers. His discovery here, as in that of fixed air, was accidental. Making experiments on the water of different temperatures, he found that the mixture would always be an arithmetical mean betwixt the two quantities mixed. Thus, on mixing water at 50 degrees with an equal quantity at 100, the temperature of the mixture would be 75 degrees; but if instead of using water only he took snow or ice for one of the quantities, the mixture was no longer an arithmetical mean betwixt the two temperatures, but greatly below it; so that a quantity of heat seemed to be totally lost and in a manner annihilated. His attention was engaged by this unexpected phenomenon, and, prosecuting his experiments, he found that, when water was converted into ice, it really became warmer than it was before; and, by keeping the fluid perfectly still during the time that cold was applied, he was able to cool it to 27 degrees of Fahrenheit’s thermometer, which is five degrees below the freezing point; but on shaking this water so cooled, it was instantly converted into ice, and the thermometer rose to 32. On reversing the experiment he found that mere fluidity in water is not sufficient to melt ice. A considerable degree of heat is necessary; and even when this is previously given to the water, the whole becomes as cold as ice by the time that the ice is melted. The result of his experiments in short was this: Water, when frozen, absorbs an hundred and thirty-five degrees of heat before its fluidity can be restored: that is, supposing a pound of ice at the temperature of 32 to be mixed with a pound of water at the temperature of 32, by adding 135 degrees, so that the temperature of the water is augmented to 167, the ice will indeed be melted, but the temperature of the whole quantity of liquid will be reduced to 32. In this case therefore the heat manifestly assumes two different modes of action: one in which it acts internally upon the substance of the body, without being sensible to the touch, while in its other state it hath no effect upon the internal parts, but affects bodies on the outside. The former state therefore the Doctor distinguished by the name of _latent_, the latter by that of _sensible_ heat.