HISTORICAL AND INTRODUCTORY n 



gives at every stage. But for years his teachings were not 

 accepted, nor were his methods followed. 



The two great books on agricultural chemistry then 

 current still belonged to the old period. Thaer and Davy, 

 while much in advance of Wallerius, the textbook writer of 

 1761, nevertheless did not realise the fundamental change 

 introduced by de Saussure ; it has always been the fate of 

 agricultural science to lag behind pure science. Thaer pub- 

 lished his Grundsdtze der rationellen Landwirtschaft in 1809- 

 1812: it had a great success on the Continent as a good, 

 practical handbook, and was translated into English as late 

 as 1844 by Cuthbert Johnson. Davy's book (79) grew out of 

 the lectures which he gave annually at the Royal Institu- 

 tion on agricultural chemistry between 1802 and 1812; it 

 was published in 1813, and forms the last textbook of the 

 older period. Whilst no great advance was made by Davy 

 himself (indeed his views are distinctly behind those of de 

 Saussure) he carefully sifted the facts and hypotheses of 

 previous writers, and gives us an account, which, however de- 

 fective in places, represents the best accepted knowledge of 

 the time, set out in the new chemical language. He does not 

 accept de Saussure's conclusion that plants obtain their carbon 

 chiefly from the carbonic acid of the air : some plants, he says, 

 appear to be supplied with carbon chiefly from this source 

 but in general he supposes the carbon to be taken in through 

 the roots. Oils are good manures because of the carbon and 

 hydrogen they contain; soot is valuable, because its carbon 

 is " in a state in which it is capable of being rendered soluble 

 by the action of oxygen and water". Lime is useful because 

 it dissolves hard vegetable matter. Once the organic matter 

 has dissolved there is no advantage in letting it decompose 

 further: putrid urine is less useful as manure than fresh 

 urine, whilst it is quite wrong to cause farmyard manure to 

 ferment before it is applied to the land. All these ideas 

 have long been given up, and indeed there never was any 

 sound .experimental evidence to support them. It is even 

 arguable that they would not have persisted so long as they 



