The final objection to his theory was one of Rey's own suggestion: Why does not earth go on increasing in weight indefinitely? Because "the thickened air attaches itself to it and continues to adhere to the most minute of its particles, so that its weight goes on increasing from the beginning to the end; but when it is all enveloped in air it can not take any more." He concludes then, and terminates his treatise by declaring with pride that he has found the real way of the truth, breaking the road for his successors, and advising them not to go astray from it.
This is the summary of the works of Jean Rey. A skillful experimenter, he knew how to use the balance, and it was the balance that suggested to him the result of his experiments. His book is a brief one. A single principal experiment is described, a single object is pursued in it. But he made two great advances in science. He discovered the weight of the air, being the first to publish that hypothesis, and verified it by experiments in chemistry and physics. The increase of the weight of lead and tin on calcination had been noticed for a long time by the alchemists, and even Galen knew of it. But nobody before Rey found that the cause of that increase in weight came from the air from that thickened and heavy air. It was certainly a remarkable achievement to announce such a fact at a time when chemistry had made so little advance. No gas was yet known; and it was not till about 1719 that a misunderstood man of science, Mortrel d'Élément, found means to decant air through water into bottles, and taught in a public lecture in Paris "how to make air visible and perceptible enough to measure it in pints or in whatever quantity you will." It was not his fault, therefore, that he did not advance further.—Translated for The Popular Science Monthly from the Revue Scientifique.