260 GRAVITY AND LEVITY OK GASES. 



most heavy. The comparative degree of den- 

 sity, and of rarity, between each, may be ascer- 

 tained in a different way ; it may be ascertained 

 by exhausting the air in the bottle, while the me- 

 dium of atmospheric air, or of any other gas, is 

 allowed to remain, external to it ; instead of the 

 rarefied air which is left in the bottle, being 

 balanced as before, it is found that it presses up- 

 wards, and is lighter ; and the number of metal- 

 lic grains which are necessary to be added to 

 it, in order that the equilibrium may be re- 

 stored is the proper measure of the difference 

 which exists, between the rarefied air in the 

 bottle, and the dense air which has been dis- 

 placed by it.* 



By means, such as these, we infer, that the 

 quantities of matter contained in different bo- 

 dies, are in the compound ratios of their magni- 

 tude and densities ; and that the different mea- 



* It ought, however, to be observed, that in measuring 

 the relative density and rarity of different bodies, by any 

 given standard that may be employed ; whether such standard 

 be solid, liquid, or gaseous, that no precise accuracy can be 

 expected, unless the standard which is employed is of the 

 same bulk, as the bulk of the matter which is to be mea- 

 sured ; for as every dense body loses as much of its weight, 

 as is equal to the bulk of matter which it displaces, the stan- 

 dard of the greatest bulk, will necessarily be lighter than the 

 one of less bulk, and vice versa. 



