Thus the Reviewers, as well as others, reasoned _a priori_, and several facts were adduced to prove that no such changes in capacity could take place. But however strong the arguments adduced, or however plain the experiments might be, little or no notice was taken of them, and the enlargement or diminution of _capacities_ has been repeated, seemingly by rote, from one author to another, without the least inquiry or investigation. Dr. Girtanner indeed says that “the oxygen united with the arterial blood in the lungs” is the cause of the _great capacity_ of the arterial blood for heat. But this is assigning a very doubtful cause for a very doubtful effect. He ought to have proved in the first place that arterial blood really has this capacity; for its being _hotter_ than the blood of the veins, only shews that it parts with more heat to surrounding bodies than venous blood does; which is a proof that it contains _less_ heat, if there be any difference, than that of the veins. But the truth is, that the _capacity_ for containing heat depends neither on the oxygenation nor hydrogenation of a fluid, but upon its density. The more fluid and the more easily expansible into vapour that any substance is, the greater quantity of heat it is capable of containing, and _vice versa_. This has been fully ascertained by Mr. William Jones, an English clergyman, whose observations on the generally received system of philosophy contain many particulars worthy of attention. From his experiments it appears that a piece of red-hot iron, thrown into water, imparts much less _sensible_ heat to it, and is itself much more effectually quenched, than by throwing it into an equal quantity of quick-silver of the same temperature with the water. As the quick-silver therefore becomes much hotter to the touch than water does upon throwing a piece of red-hot iron into it, and as the iron itself is much more imperfectly quenched by the metal than by the water, it follows that the latter is capable of containing much more heat than the former. But such experiments are not applicable to the blood. Though that of the arteries may be somewhat hotter than the venous blood, yet the reason is obvious. The heat is communicated directly to the arterial blood in the lungs; but during the circulation a part of it evaporates, and the farther distant any part is from the lungs, the more cool will the vital fluid be, without regard to any alteration of _capacity_, which indeed never can be shown to exist.