ATMOSPHERIC MATTER. 387 



sion and balance, to precipitation and fall, are 

 merely different stages and degrees of one and 

 the same process ; or, in other words, the state 

 of levity which the watery particles had ao 

 quired, while they subsisted in a gaseous state* 

 become progressively impaired, or lost ; they 

 become relatively heavier than the rare medium 

 in which they are situated ; they therefore obey 

 the laws of relative weight, and in their descent 

 through the air, they become compressed and 

 shaped in different forms, increasing in magni- 

 tude as they approach the earth ; they fall on 

 the surface, and are known by the various ap 

 pellationsof rain, of hail> and of snow. These 

 spontaneous changes, which take place in the 

 atmosphere, may in a limited degree be iim^ 

 tated by art, and are followed by the same 

 consequences. The expansibility of the atmos- 

 pherical air may be weakened by combustion, 

 as well as by being rarified and partly exhaust- 

 ed, as in the air-pump ; such is the dilatation 

 which, the residuary air has in consequence 

 sustained, that its power of supporting the 

 aqueous particles becomes progressively and 

 proportionably weakened : after a few strokes 

 of the piston, a very sensible change in the 

 decomposition of the air becomes percepti- 

 ble ; from being transparent and dry, it be- 

 comes turbid and moist ; and when the exhaus- 

 tion is made as complete as it is st*sceptible of, 

 cc 2 



