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250 who say that this would be done to avoid a vacuum do not say much. They point to the final cause, and this demonstration concerns the efficient one, which rules that there can not be a vacuum." Key's book, however, while it marked an immense progress in science, contains some considerable errors. Thus, he undertakes in his fifth essay to show, by the acceleration of the motion of falling bodies as they descend, that fire and air are heavy, and he attempts to prove by an ingenious but fallacious demonstration that they are forced down more and more rapidly by the increasing weight of the air and fire above them.

Having tried to "impress upon everybody's heart that air has weight," Rey announced the proposition, then new to science, deduced from this principle, that "weight is so closely joined to the primal matter of the elements, that changing from one to another they always keep the same weight. The weight which each portion of matter takes from its cradle it will carry to its grave. In whatever place, under whatever form, and to whatever volume it may be reduced, always the same weight." This principle, which he discovered more than a century before Lavoisier, was confirmed by a curious experiment of Brun's, who in 1644, having constructed a distilling apparatus, hermetically sealed, inclosed within it wood of guaiac, box, or oak, weighed the whole, and distilled it. The wood was destroyed; but a new weighing, made at the end of the experiment, showed that the total weight of the apparatus had not changed during the distillation. The experiment was a delicate one, and proves that they knew how to work in Rey's time. Mistakenly believing that water could change into air, Rey constructed an apparatus for determining what volume of air a given quantity of water would form. It consisted of a bulb (æolipile) in which water could be boiled, connected by a tube with a cylinder open at the top; a piston was worked in this cylinder. The piston was brought down to the bottom of the cylinder. Heat was applied to the æolipile and the water was made to boil or was "transformed into air"; the piston of course rose, under pressure of the steam or "water air," and the capacity of the part of the cylinder below it showed the volume of the air that was supposed to be formed. Then the æolipile could be removed, the opening from it into the cylinder stopped up, and the cylinder exposed to cold, when the piston would be forced down and the vapor frozen or turned into water. Unfortunately for himself, Rey did not personally try this experiment, or he might have anticipated Papin by half a century. But not more than a year after the publication of his essays—September 1, 1631—Père Mersenne said: "As to the experiments with the æolipile, I have made them; but it is a false imagination to suppose that the water which issues from