[73] Nitrous air is that suffocating vapour which arises when aqua fortis is poured upon metals. When taken into the lungs it destroys animal life more quickly than any other species. On reading the Doctor’s account of this experiment; it must be very obvious, that, however decidedly he may be of opinion that it proves _beyond a doubt_ the existence of hydrogen air in the venous blood, yet there is not one solid reason; from what he says, for supposing any such thing. How can any man determine from the mere _smell_ of _sheep’s_ blood taken out of the body of the animal, and mixed with a poisonous vapour, what is the composition of _human_ blood in the living body? In the case of any substance suspected to contain elastic air, the air-pump will always afford an _experimentum crucis_. But we know that venous blood does not yield any elastic vapour by the pump: if instead of blood, however; we should fill a portion of vein with beer, cyder, or other fermented liquor, it would instantly discover, by its swelling up, that it really contained air in an elastic state. If then from the tumefaction of the vein when filled with fermented liquor we conclude that the latter contains fixed air, why should we not, from the non-tumefaction of it when filled with blood, conclude that the vital fluid contains no air? If Dr. Girtanner was so well assured that the venous blood contains hydrogen air, he ought to have expelled some of it from a portion of the blood, noted the difference between the blood which had lost its air, and that which had not, and then, by adding the air to it again, restored the blood to its former state. Nothing less then recomposition can prove the truth of a chemical analysis; as division can only be proved by multiplication, or multiplication by division.