108 ON THE FOOD OF PLANTS. 



" finally resolved. Van Helmont, in 1610, con- 

 " ceived that he had proved by a decisive expe- 

 " riment, that all the products of vegetables were 

 " capable of being generated from water. His 

 ** results were shown to be fallacious by Wood- 

 •' ward, in 1691. But the true use of water in 

 *' vegetation was unknown till 1785, when Mr. 

 *' Cavendish made the grand 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 enquirers in the six- 

 " teenth and seventeenth centuries formed some 

 '' happy conjectures respecting its real nature. 

 *' Sir Kenelm Digby, in 1660, supposed that it 

 " contained some saline matter, which was an 

 ** essential food of plants. Boyle, Hooke, and 

 " Mayow, between 1665 and 1680, stated that 

 " a small part of it only was consumed in the 

 " respiration of animals, and in the combustion 

 " of inflammable bodies. But the true statistical 

 *' analysis of the atmosphere is comparatively a 

 *' recent labour, achieved towards the end of 

 " the last centuiy, by Scheele, Priestley, and 

 *' Lavoisier. These celebrated men showed that 

 " its principal elements are two gases, oxygene 



