or Mayo, John, a medical man of some eminence, but more remarkable for his curious speculations on some chemical subjects, was born in Cornwall in 1645. His most interesting work is a short Latin treatise, *De Sole Nitro, et Spiritu Nitro-aereo*, which appeared at Oxford in 1674. In this he adopted the theory of combustion of Dr Hooke, which had appeared about ten years before; and he added some ingenious original experiments, in which he appears to have anticipated some far more recent investigations on the weight gained by metals on calcination; and he maintained that atmospheric air underwent a change in composition during the combustion of fuel. The apparatus he employed in these experiments was ingenious, and not very different from that of modern chemistry. But the attention of chemists was drawn away from the simple explanations of Hooke and Mayow by the fanciful speculations of Stahl regarding phlogiston. In the 14th chapter of his little treatise we find that the ideas of Mayow on chemical affinity were much more accurate than those of any of his contemporaries; and we must regard him as one of the pioneers of modern chemistry, though some of his views are undoubtedly erroneous, as might be expected from the period in which he lived. He died at Bristol in 1679, at the early age of thirty-four.