with a burning glass, undergoes an increase of weight which can be attributed to nothing else but these particles. Mayow argued that the same particles are consumed in respiration, because he found that when a small animal and a lighted candle were placed in a closed vessel full of air the candle first went out and soon afterwards the animal died. However, if there was no candle present the animal lived twice as long. He concluded that this constituent of the air is absolutely necessary for life, and supposed that the lungs separate it from the atmosphere and pass it into the blood. It is also necessary, Mayow inferred, for all muscular movements, and he thought there was reason to believe that the sudden contraction of muscle is produced by its combination with other combustible (salino-sulphureous) particles in the body; hence the heart, being a muscle, ceases to beat when respiration is stopped. Animal heat also is due to the union of nitro-aerial particles, breathed in from the air, with the combustible particles in the blood, and is further formed by the combination of these two sets of particles in muscle during violent exertion. In effect, therefore, Mayow – who also gives a remarkably correct anatomical description of the mechanism of respiration – preceded Priestley and Lavoisier by a century in recognising the existence of oxygen, under the guise of his "spiritus nitro-aereus," as a separate entity distinct from the general mass of the air. Mayow perceived the part "spiritus nitro-aereus" plays in combustion and in increasing the weight of the calces (oxides) of metals as compared with metals themselves. Rejecting the common notions of his time that the use of breathing is to cool the heart, or assist the passage of the blood from the right to the left side of the heart, or merely to agitate it, Mayow saw in inspiration a mechanism for introducing oxygen into the body, where it is consumed for the production of heat and muscular activity. He even vaguely conceived of expiration as an excretory process. Using bell-jars over water Mayow showed that the active substance that we today call oxygen constitutes about a fifth part of the air. Parts of Mayow's work seem to agree with modern ideas regarding air and combustion. Mayow noted, as had others before him, that some materials gain weight on strong heating. Antoine Lavoisier (1743–1794) and others