THE ESSAYS OF JEAN RET. 251 



it is turned into air; it still remains water, and will naturally 

 return to itself." 



Resuming his study concerning the weight of the air, Rey 

 observed that his predecessors had failed to find it because they 

 had weighed the air in itself : " Balancing the air in the air itself, 

 and not finding weight, they have thought that it had none. But 

 let them balance water (which they know is heavy) in water 

 itself, and they will find no weight there too ; it being a fact that 

 no element has weight when balanced in itself. Everything that 

 is weighed in the air and everything that is weighed in water 

 should, for an equal volume, have as much more weight as it has 

 more matter than the air or the water in which the balancing is 

 done." Air, he said, could be made heavy by mixing it with 

 some foreign matter having more weight; by compressing its 

 particles; or by removing the lighter portions. In demonstra- 

 tion of the first principle, Rey determined by experiment that 

 moist or cloudy air was heavier than dry air ; of the second, he 

 showed that, if a globe was filled by a strong draft of air from a 

 bellows, it would be heavier than the same globe " empty." He 

 even tried to make use of compressed air in the construction of a 

 wind arquebus, but he did not carry out his idea ; and the honor 

 of making this invention practical belongs to the Sieur Marin 

 Bourgeois, of Lisieux. Inversely, Rey observed that if one takes 

 a glass vial cold, warms it a little on a chafing dish, and weighs 

 it, he will find that it weighs less, because air has gone out from 

 it ; and in order to find how much, the pipe should be put, still 

 warm, into water, which it will suck up till as much water comes 

 into it as air has gone out of it. Rey was, however, not the first 

 who had observed this fact, for Drebbel had anticipated him. 

 The converse of these principles was also enunciated by Rey, viz., 

 that the weight of air may be diminished by purifying it from 

 heavier foreign matter, by extending it to ampler limits, and by 

 extracting its heavier parts. " Even the balance sometimes de- 

 ceives " ; for, " if we examine the balances, cases may be found in 

 which the object weighed will appear heavier or lighter without 

 adding or subtracting foreign matter ; as when it has been con- 

 tracted or expanded." In support of this view, Rey cited as ex- 

 amples the cases of a ball of feathers tightly tied up, which will 

 weigh more than the same feathers loose ; and of two ingots, one 

 of gold and the other of iron, which will balance one another 

 without having the same absolute weights, "for the gold occupies 

 a smaller volume for equal weight, and consequently displaces 

 less air." These views were confirmed about 1650 by the inven- 

 tions of Otto von Guericke. 



Rey was now able to answer the question put to him by 

 Brun, and to explain the cause of the increase of weight shown 



