250 THE POPULAR SCIENCE MONTHLY. 



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 prin- 

 ciple, 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 (seolipile) 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 seolipile 

 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 

 seolipile 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 Pere Mersenne said : " As to the 

 experiments with the seolipile, I have made them; but it is a 

 false imagination to suppose that the water which issues from 



