Priestley and Cavendish had thus distinctly established the existence of three kinds of air, viz. dephlogisticated air, phlogisticated air, and inflammable air. Cavendish had shown that when the last named is exploded with common air water is produced (which is composed of dephlogisticated and inflammable airs), and phlogisticated air remains. Common air had thus been proved to consist of these two--phlogisticated and dephlogisticated airs (nitrogen and oxygen). Applying these results to the phenomenon of the calcination of metals, Cavendish gave reasons for thinking that the metals act towards common air in a manner analogous to that in which inflammable air acts--that they withdraw dephlogisticated and leave phlogisticated air; but, as he was a supporter of the phlogistic theory, he rather preferred to say that the burning metals withdraw dephlogisticated air and phlogisticate that which remains; in other words, while admitting that a metal in the process of burning gains dephlogisticated air, he still thought that the metal also loses _something_; viz. phlogiston. That Cavendish in 1783-84 had proved air to consist of two distinct gases, and water to be produced by the union of two gases, must be remembered as we proceed with the story of the discoveries of Lavoisier. * * * * * ANTOINE LAURENT LAVOISIER, born in Paris in 1743, was the son of a wealthy merchant, who, judging from his friendship with many of the men of science of that day, was probably of a scientific bent of mind, and who certainly showed that he was a man of sense by giving his son the best education which he could obtain. After studying in the Mazarin College, Lavoisier entered on a course of training in physical, astronomical, botanical and chemical science. The effects of this training in the accurate methods of physics are apparent in the chemical researches of Lavoisier. At the age of twenty-one Lavoisier wrote a memoir which gained the prize offered by the French Government for the best and most economical method of lighting the streets of a large city. While making experiments, the results of which were detailed in this paper, Lavoisier lived for six weeks in rooms lighted only by artificial light, in order that his eyesight might become accustomed to small differences in the intensities of light from various sources. When he was twenty-five years old Lavoisier was elected a member of the Academy of Sciences. During the next six years (1768-1774) he published various papers, some on chemical, some on geological, and some on mathematical subjects. Indeed at this time, although an ardent cultivator of natural science, he appears to have been undecided as to which branch of science he should devote his strength.