160 GRAVITY OF VAPOUR. 
previously existing, and thus perplexes the re- 
sult. Hence arises the necessity, when we in- 
vestigate the specific gravity of vapour, of ope- 
rating upon it when perfectly secluded from | 
contact with its appropriate liquid. 
In determining the specific gravity of any 
elastic fluid, four things are essentially neces- 
sary to be ascertained: viz. the volume,—the 
pressure,—the temperature, and the weight. The 
specific gravity is the weight relatively to the 
weight of some other standard elastic fluid when 
both are taken of the same voluwme,—pressure, 
and temperature. The effects of volume, pres- 
sure, and temperature upon the weight are well 
known and can be allowed for when one or all 
of these three data vary from the standard, as in 
practice frequently occurs. The vapours of 
water, alcohol, ether, pyroxilic spirit, and acetic 
acid are all compounds ; the first of oxygen and 
hydrogen, the last three of oxygen, hydrogen 
andcarbon. The specific gravities of the three 
first-named vapours have been determined by 
Gay Lussac and by other eminent chemists, 
with whose results I have found my own to 
agree, except that in general I have made the 
vapours a little heavier than their results—The 
differences may possibly arise from errors of 
