1. Nobody can reasonably suppose that hydrogen air is the cause of the dark colour of the blood in the veins, because there is no source from which it can be derived; and, besides, it is certain that no kind of air can exist in its elastic state in the blood, without destroying the life of the animal. Some experiments proving this are given by Dr. Girtanner himself. It is true that an aerial vapour, of the nature of _fixed air_, exhales from the body by insensible perspiration; but there can be no doubt that this receives its elasticity only at the surface of the body, and is expelled the moment it is formed. It has indeed been proved, by undeniable experiment, that no air of any kind exists in the larger veins; because a portion of a vein, included between two ligatures, being cut out, and put under the receiver of an air-pump, does not swell in the least when the air is exhausted, which yet must be the case, did the smallest quantity of elastic air exist in it.[69] [69] _Hydrogen_ air is the same with that by Dr. Priestley called _inflammable_ air. He also discovered the true composition of it. Having included a few grains of charcoal in the receiver of an air-pump, and exhausted the air, he heated it in vacuo by means of a large burning glass. The charcoal was entirely volatilized and converted into this kind of air. He found, however, that without some small portion of moisture this volatilization did not take place. 2. When the Doctor asserts that the dark colour of the venous blood is owing to the carbon it contains, he is in the first place chargeable with the error of former chemists, who supposed that every thing which could be extracted from any substance by fire, existed previously in it, in that very form in which it is extracted by the fire; and in the second place he speaks entirely at random, without even a shadow of proof. Nay, he himself tells us, that he has repeated two of Dr. Priestley’s experiments, which in the clearest manner demonstrate, that neither the addition nor the abstraction of carbon, or any thing else, give this dark colour to the venous blood. “A small glass tube (says he) filled with arterial blood, of a bright vermilion, was sealed hermetically,[70] and exposed to the light. The blood changed its colour by degrees, and in six days became black as venous blood. The same experiment was repeated, with this difference only, that the tube was exposed to heat, and not to the light. The blood became black in a shorter time.” In these experiments it is plain, that if the blood contained oxygen at first, it did so at the last; the same with regard to carbon. How came it then to pass, that without either evaporation of the former, or addition of the latter, the change should be produced? If the oxygen imbibed by the blood in the lungs was sufficient to produce the red colour, why did it not preserve it? The case here is precisely similar to what happens with the calx of silver. When that