known till 1785; when Mr. Cavendish made the 
discovery, that it was composed of two elastic 
fluids or gases, inflammable gas or hydrogene, and 
vital gas or oxygene. 
Air, like water, was regarded as a pure element 
by most of the ancient philosophers: a few of the 
chemical inquirers in the sixteenth and seventeenth 
centuries, formed some happy conjectures respect- 
ing its rejal nature. Sir Kenelm Digby in 1660, 
supposed that it contained some saline matter, 
which was an essential food of plants. Boyle, 
Hook, and Mayow, between 1665 and 1680, 
stated, that a small part of it only was consumed 
in the respiration of animals, and in the combus- 
tion of inflammable bodies ; but the true statical 
analysis of the atmosphere is comparatively a 
recent labour, achieved towards the end of the 
last century by Seheele, Priestley, and Lavoisier. 
These celebrated men shewed that its principal 
elements are two gases, oxygene and azote, of 
which the first is essential to flame, and to the life 
of animals, and that it likewise contains small quan- 
tities of aqueous vapour, and of carbonic acid gas ; 
and Lavoisier proved that this last body is itself a 
compound elastic fluid, consisting of charcoal dis- 
solved in oxygene. 
Jethro Tull, in his treatise on Horse-hoeing, 
published in 1733, advanced the opinion, that 
minute earthy particles supplied the whole nou- 
rishment of the vegetable world ; that air and water 
were chiefly useful in producing these particles 
from the land ; and that manures acted in no other 
way than in ameliorating the texture of the soil, 
