428 History of the English Landed Interest. 



combustion of certain substances in air, had discovered an 

 irrespirable gas wliicli was neither fixed air nor carbonic 

 acid, and though he termed it azote, he had in reality stumbled 

 across nitrogen. Indeed, often when Priestley discussed the 

 properties of phlogiston he was really unconsciously alluding 

 to those of nitrogen. Consequently, he it was who started 

 the theory that phlogiston was the food of plants, and 

 sought to prove from this phenomenon the enormous import- 

 ance of air to vegetable life. Arthur Young having read his 

 five volumes of Exjjer'imenfs and Observafions ^ with avidit}'', 

 and having followed with intelligent interest the experiments 

 of Dr. Ingenhous on vegetation, set to work upon his own 

 account to put to the rude test of practical experiment the 

 conflicting theories which he had thus imbibed. Priestley had 

 contended that though phlogiston was the essence of plants 

 and animals, they were pov/erless to extract it from many of 

 its sources, and were liable to be injured by any excessive 

 artificial application of it to their delicate organizations. 

 Phlogiston, as a result of putrefaction in intimate contact 

 with the roots, or absorbed from the air by the leaves, was as 

 essential to vegetable life as dephlogisticated air was to animal 

 life ; and the extraction of the former element by the lungs of 

 plants left the atmosphere purer for animal respiration than 

 it would otherwise have been. In that which Priestlej'' and 

 Ingenhous therefore understood by the phlogiston of the air 

 we recognise the combined properties of nitrogen and carbonic 

 acid, and in the somewhat antagonistic results obtained from 

 their experiments with vegetable growth we prove their in- 

 ability to individualise these two constituents of the atmo- 

 sphere. The supposition that for plants to assimilate phlogis- 

 ton by their roots the putrefaction of those compounds con- 

 taining it became a necessity, had led chemists to sanction 

 the common practice of frequently turning the muck heap, but 

 on this error the practical Young now brought the light of his 

 valuable experience to bear. He had found that the manure 

 from his covered sheep-yards was worth double the quantity 



' Experiments and Observations relative to various Branches of Nat. 

 Philos., vol. ii. p. 1. 



