That great naturalist, the Honourable Robert Boyle (born in 1626, died in 1691), very perseveringly besought those who examined processes of calcination to pay heed to the action of everything which might take part in the processes. He was especially desirous they should consider what part the air might play in calcinations; he spoke of the air as a "menstruum or additament," and said that, in such operations as calcination, "We may well take the freedom to examine ... whether there intervene not a coalition of the parts of the body wrought upon with those of the menstruum, whereby the produced concrete may be judged to result from the union of both." It was by examining the part played by the air in processes of calcination and burning that men at last became able to give approximately complete descriptions of these processes. Boyle recognised that the air is not a simple or elementary substance; he spoke of it as "a confused aggregate of effluviums from such differing bodies, that, though they all agree in constituting by their minuteness and various motions one great mass of fluid matter, yet perhaps there is scarce a more heterogeneous body in the world." Clement of Alexandria who lived in the end of the 2nd, and the early part of the 3rd, century A.D., seems to have regarded the air as playing a very important part in combustions; he said--"Airs are divided into two categories; an air for the divine flame, which is the soul; and a material air which is the nourisher of sensible fire, and the basis of combustible matter." Sentences like that I have just quoted are found here and there in the writings of the earlier and later alchemists; now and again we also find statements which may be interpreted, in the light of the fuller knowledge we now have, as indicating at least suspicions that the atmosphere is a mixture of different kinds of air, and that only some of these take part in calcining and burning operations. Those suspicions were confirmed by experiments on the calcination of metals and other substances, conducted in the 17th century by Jean Rey a French physician, and by John Mayow of Oxford. But these observations and the conclusions founded on them, did not bear much fruit until the time of Lavoisier, that is, towards the close of the 18th century. They were overshadowed and put aside by the work of Stahl (1660-1724). Some of the alchemists of the 14th, 15th and 16th centuries taught that combustion and calcination are processes wherein _the igneous principle_ is destroyed, using the word "destroyed" in its alchemical meaning. This description of processes of burning was much more in keeping with the ideas of the time than that given by Boyle, Rey and Mayow. It was adopted by Stahl, and made the basis of a general theory of those changes wherein one substance disappears and another, or others, very unlike it, are produced.