10 THE SCIENTIFIC PRINCIPLES 



Each year, for ten successive years, from 1802 to 



181 2, he delivered a course of lectures which, in 



18 1 3, were published in a work entitled, 'Elements 

 of Agricultural Chemistry/ which he finally revised 

 for the fourth edition in 1827, but which has gone 

 through several editions since. 



In those lectures Sir Humphrey Davy passed in 

 review and correlated the then existing knowledge, 

 both practical and scientific, bearing upon agriculture. 

 He treated of the influences of heat and light ; of the 

 organisation of plants ; of the difference, and the 

 change, in the chemical composition of their different 

 parts ; of the sources, composition, and treatment of 

 soils ; of the composition of the atmosphere, and its 

 influence on vegetation ; of the composition and the 

 action of manures ; of fermentation and putrefaction ; 

 and, finally, of the principles involved in various 

 recognised agricultural practices. 



Concurrently with the delivery and the publication 

 of Davy's Lectures in England, the most prominent 

 writer on the scientific principles of agriculture on 

 the continent was Thaer. In 18 10 he published a 

 work entitled ' Principes Eaisonnes d' Agriculture/ 

 and for some years afterwards contributed papers on 

 some special points. He considered that the fertility 

 of the soil depended on the amount and character 

 of the humus it contained ; it being, in his view, the 

 only substance, excepting water, that yielded aliment 

 to plants. He pointed out that it was the residue 

 of previous vegetation, containing carbon, hydrogen, 

 oxygen, and nitrogen, associated with phosphorus, 

 sulphur, and some salts. He quotes De Saussure 

 as having shown that humus contains a lower per- 

 centage of oxygen and higher percentages of carbon 

 and nitrogen than the plants from which it has been 



