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 tpiritus nitro-aereu. 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, the entrance into which of air from without be prevented, you will see in a short time the candle go out, nor will the animal long survive its funeral torch. Indeed, [says Mayow] I have found by observation that an animal shut up in a flask together with a candle will continue to breathe for not much more than half the time than it otherwise would, that is, without the candle." Something contained in the air, necessary alike for supporting combustion and for sustaining life, passes from the air into the blood. Mayow expressed his thoughts in these words: "And indeed it is very probable that certain particles of a nitro-saline nature, and those very subtle, nimble, and of very great fermentative power, are separated from the air by the aid of the lungs and introduced into the mass of the blood. And so necessary for life of every kind is that aereal salt (constituent) that not even plants can grow in earth the access of air to which is shut off. But if that same earth be exposed to air and so forthwith impregnated with that fecundating salt, it at once becomes fit again for growing."1 Mayow fully appreciated the importance of his nitro-aereal particles in the processes of life; he had indeed a fairly accurate conception of a sound theory of animal heat; he saw that they were equally necessary for burning, or combustion, and for respiration, and so was enabled to draw a parallelism between the two processes; he pointed out that they were essential for the ordinary activity of the muscles of the body, that as muscle work was increased more particles from the air were required; indeed, he clearly foresaw the need which the body had for these igneo-aereal particles in all the chemical processes of life. And thus was foreshadowed a conception of oxidation, a hundred years before Priestley evolved his phlogiston theories and Lavoisier discovered oxygen.

The discoveries of Lavoisier, published in 1789, led to a clear understanding of combustion as a process of oxidation, and paved the way for a fuller knowledge of the part played by the oxygen of the air in the chemical reactions going on in the animal body. Lavoisier showed that the oxygen drawn into the lungs with the air breathed was used in the body for the oxidation of certain substances, carbon being transformed thereby into carbon dioxide, and hydrogen into water. Further, he noted that these oxidations were carried forward on a large scale, and he emphasized the importance of oxygen as being the true cause of the varied decompositions taking place in the living body. The larger the amount of oxygen inspired, the more extensive the oxidation, and consequently the rate of respiration as modifying the intake of oxgyen served in his opinion as a regulator to control the extent of the oxidative processes. He pointed out that a definite relationship existed between the amount of work done by the body and the oxygen consumed; greater muscular activity, lower temperature of the surrounding air, the activities attending the digestive functions, all seemed to be associated with a greater utilization of oxygen. Oxidation was the pivot around which all the chemical reactions of the body seemed to centre. Lavoisier, however, was not a physiologist, and he was, quite naturally perhaps, led into some errors. For example, he considered that the process of combustion or oxidation took place in the lungs, certain fluids rich in carbon and hydrogen formed in the different organs of the body being brought there for exposure to the inspired oxygen. Further, his views implied a simple and complete combustion, in which complex substances rich in carbon were directly and completely oxidized to carbon dioxide and water, in much the same manner as combustion occurs outside the body. Again, he assumed that the amount of oxygen taken into the lungs determined the extent of oxidation, just as the use of the bellows, by increasing the draft of air, causes the fire to burn more brightly.

1 Quoted from Foster's Lectures, p. 105.