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 demonstration 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 deceives"; 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 contracted or expanded." In support of this view, Rey cited as examples 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 inventions 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
