vigorous argument, he contends that the “air taken in by breathing is the chief cause of the life of animals, far more essential than the working of the heart and the circulation of the blood.” He quotes the experiments of Boyle, who showed in 1660 “that even in a partial vacuum brought about by his air pump, flame was extinguished and life soon came to an end; the candle went out and the mouse or the sparrow died.” [15] Taken from Sir Michael Foster’s “Lectures on the History of Physiology during the Sixteenth, Seventeenth, and Eighteenth Centuries.” Cambridge, 1901, p. 12. [16] See Foster’s Lectures, p. 136. At this time, and for long afterwards, the belief was prevalent that the air taken up by the blood in the lungs was the air of the atmosphere in its entirety. No one appears to have thought of the possibility of only a part of the air being used, for at that time there was no suspicion that air was a mixture of substances. Mayow, however, in 1668, showed that it was not the whole air which was employed for respiration, but a particular part only. At this time, great attention was being given to a study of nitre or saltpetre; its wonderful properties in combustion were being recognized, and Mayow, who was a chemist of repute, claimed that it had its origin partly in the air and partly in the earth. The air “which surrounds us, and which, since by its tenuity escapes the sharpness of our eyes, seems to those who think about it to be an empty space, is impregnated with a certain universal salt, of a nitro-saline nature, that is to say, with a vital, fiery, and in the highest degree fermentative spirit,” to which the name of “igneo-aereus” was applied. Nitre was shown to be composed of a _sal fixum_ or sal alkali,--potash as it is now called,--and was obviously derived from the earth, while the other part of nitre was made up of the _spiritus acidus_, or nitric acid. For a time it was supposed that the whole of this _spiritus acidus_ was contained in the atmosphere, but it was soon recognized that this could not be the case, since nitric acid was found to be a corrosive liquid, destructive to life and quite incapable of supporting combustion. Hence, Mayow concluded that only a part of the acid exists in the atmosphere, viz., that part which he termed _spiritus nitro-aereus_. In combustion, there is something in the air which is necessary for the burning of every flame, unless perchance igneo-aereal particles should pre-exist in the thing to be burnt. These igneo-aereal particles form “the more active and subtle part of air which is thus necessary for combustion, exist in nitre and indeed constitute its ‘more active and fiery part.’” Mayow fully recognized that burning and breathing involved in a measure the same process; both consisted in the consumption of the igneo-aereal particles present in the air. “If a small animal and a lighted candle be shut up in the same vessel,