OF AGRICULTURE. 7 



soil, either from its own natural resources, or by 

 means of manure. It will be seen, however, from 

 the facts just stated, how very recently fixed ideas 

 on the subject have been arrived at. 



Then, as to the combustible or volatile constituents 

 which are expelled in the incineration — the carbon, 

 the hydrogen, the oxygen, and the nitrogen. From 

 what we now know of the sources of these con- 

 stituents of plants, it is obvious that a knowledge of 

 the composition of the atmosphere and of water was 

 essential to any true conception of the main features 

 of the vegetative process, and it was only towards 

 the end of the last century that the composition of 

 the air, and of water, and their mutual relations with 

 vegetation, were first pointed out. 



I shall have to go into these matters in some 

 detail in subsequent lectures, but I would here 

 observe that it is to the collective labours of Black, 

 Scheele, Priestley, Lavoisier, Cavendish, and Watt, 

 that we owe the knowledge that common air consists 

 of nitrogen and of oxygen, with a little carbonic acid ; 

 that carbonic acid itself is composed of carbon and 

 of oxygen ; and that water is composed of hydrogen 

 and oxygen ; whilst Priestley and Ingenhousz, Sen- 

 nebier and Woodhouse, investigated the mutual re- 

 lations of these bodies and vegetable growth. 



Thus, Priestley observed that plants possessed the 

 faculty of purifying air vitiated by combustion, or 

 by the respiration of animals ; and he having dis- 

 covered oxygen, it was found that the bubbles which 

 Bonnet had shown to be emitted from the surface of 

 leaves immersed in water consisted chiefly of that 

 gas. Ingenhousz demonstrated that the action of 

 light was essential to the development of these 

 phenomena, and Sennebier proved that the oxygen 



