Translation of Rey's, Essaj/s. 269 



change which causes it to become heavier ? I remark, that it 

 may happen in three different ways ; either by the mixture of 

 some heavier foreign matter ; by the compression of its parts ; 

 or by the separation of its lighter portions. Let us speak, first, 

 of the first, and then of the two others. It is certain that the air 

 is capable of containing many matters heavier than itself; such 

 are the vapours and exhalations which rise from the water, or 

 the earth. A portion of air imbibed with these matters, will 

 weigh more than an equal portion of another air containing 

 nothing of the kind ; like as sea- water is heavier than the 

 water of fresh rivers ; the former containing much salt, which 

 the latter is free from. Observe, I pray you, how, in cloudy 

 weather, at first opening your high windows, the air enters your 

 chamber loaded with fog ! Do you not conclude that this weighs 

 more than the other, since it cleaves it and falls down in it? 

 Fill a balloon with this cloudy air, it will weigh more than the 

 same filled with pure unmixed air. Reason accords with this 

 experiment, saying thus ; if to two equal weights, we add two 

 unequal weights, the two weights will be unequal, and that 

 will be the greatest, to which the greatest weight has been 

 added. If we take, for instance, two portions of the same air, 

 each equal to ten cubic inches, and add to one of them two 

 inches of water, and to the other two inches of air, who but 

 perceives that these two portions will be very equal in volume, 

 but unequal in weight, and that the one containing the water 

 will be the heaviest? This is so manifest, that I abstain from 

 saying any thing more about it, especially as this mode of in- 

 creasing weight, has not much to do with our subject. Proceed 

 we, therefore, to the others. 



Essay X. 



Air is rendered heavy by the compression of its parts. 



The second mode by which air increases in weight, is by 

 the compression of its parts ; for nature has willed, for reasons 

 known to herself, that the elements should dilate and contract, 

 within certain limits, which she has prescribed to them. 

 Within this space, we see a portion of an element now narrowly 

 contracted, now widely distended. Observe the pot, lialf full 



