RELATION OF VAPOUR AND All*. ">")7 



density bear to the pressure ? The use which had 

 been made of tubes containing columns of mercury 

 by which the pressure or portions of air was varied 

 and measured, suggested obvious modes of devis- 

 ing experiments by which this question might be 

 answered. Such experiments accordingly were 

 made by Boyle about 1650; and the result at which 

 he arrived was, that when air is thus compressed, 

 the density is as the pressure. Thus if the pres- 

 sure of the atmosphere in its common state be 

 equivalent to 30 inches of mercury, as shown by 

 the barometer ; if air included in a tube be pressed 

 by 30 additional inches of mercury, its density 

 will be doubled, the air being compressed into one 

 half the space. If the pressure be increased three- 

 fold, the density is also trebled; and so on. The 

 same law was soon afterwards (in 1676) proved 

 experimentally by Mariotte. And this law of the 

 air's elasticity ; that the density is as the pressure, 

 is sometimes called the Boolean Law, and some- 

 times the Law of Boyle and Mariotte. 



Air retains its aerial character permanently ; 

 but there are other aerial substances which appear 

 as such, and then disappear or change into some 

 other condition. Such are termed vapours. And 

 the discovery of their true relation to air was the 

 result of a long course of researches and specula- 

 tions (SA). 



