It is impossible to avoid being impressed with the clearness and justice of Mayow’s inferential reasoning. All that was wanting was the discovery of oxygen and carbon dioxide, and the identification of the first with his fire-air, and of the second with one of the products of combustion. But these discoveries were not made until a century after his death. Had he lived, there can be little doubt that, unless discouraged by the want of appreciation with which his ideas were received, he would have continued to labour in the fruitful fields from which he had already reaped so rich a harvest. Before leaving the seventeenth century, it is perhaps fitting to mention the name of Jean Rey, a French physician, who wrote in 1630 concerning the gain in weight of tin and lead when calcined. While Rey exhibited some leaning towards the modern methods of experimentation, he still lay fettered in the bonds of mediæval scholasticism. In discussing the weight of air and fire, he finds occasion to consider the question whether a vacuum can exist. His words are so quaint that they are worth quoting: “It is quite certain that in the bounds of Nature a vacuum, which is nothing, can find no place. There is no power in Nature from which nothing could have made the universe, and none which could reduce the universe to nothing: that requires the same virtue. Now the matter would be otherwise if there could be a vacuum. For if it could be here, it could also be there; and being here and there, why not elsewhere? and why not everywhere? Thus the universe could reach annihilation by its own forces; but to Him alone who could make it is the glory of being able to compass its destruction.” And since air cannot be drawn down by a vacuum, it must descend by virtue of its own weight when it fills a hole. And hence, as air has weight, tin and lead gain in weight when they combine with air. It will be admitted that this is very inferior to the speculations and deductions of Boyle and Mayow.