“That the oxygen and nitrogen of atmospheric air exist in chemical union, appears almost demonstrable from the following evidences. “1st. The equable diffusion of oxygen and nitrogen through every part of the atmosphere, which can hardly be supposed to depend on any other cause than an affinity between these principles. “2dly. The difference between the specific gravity of atmospheric air, and a mixture of 27 parts oxygen and 73 nitrogen, as found by calculation; a difference apparently owing to expansion in consequence of combination.” These “evidences” had already been adduced by others, as stated by Davy; the first was subsequently disproved by Dalton, the second was based on inaccurate analyses of air. To these Davy added two other “proofs” which originated with him:-- “3dly. The conversion of nitrous oxide into nitrous acid, and a gas analogous to common air, by ignition. “4thly. The solubility of atmospheric air undecompounded.” Of these it may be stated that the first is invalid, and the second not true. Nitrous oxide may, under certain circumstances, give rise to a mixture of oxygen and nitrogen, but not necessarily in the same proportion as in common air; and the air boiled out from water has not the same composition as atmospheric air. Davy a few years afterwards obtained much clearer views as to the real nature of the atmosphere, and was, in fact, one of the earliest to recognise that it is merely a mixture of oxygen and nitrogen. The third section consists of an account of observations on the action of nitrous oxide upon animals, and an investigation of the changes effected in it by respiration; whilst the fourth and last gives a history of the respirability and of the extraordinary effects of nitrous oxide, with details of experiments on its powers made by different individuals. The last portion of the inquiry--in time of execution the first--is particularly interesting to the biographer of Davy, not only because the work in it was originated and carried out by him, but also from the light it incidentally throws on his character and genius:--