278 DISCOVERY CH. 



men whose scientific work has led to it are mostly 

 unknown to the politicians and other people who benefit 

 by it. 



Before the knowledge of chemistry had sufficiently 

 advanced to provide a basis for a theory of nutrition of 

 the plant, all observation of the good effect of this 

 or that substance on the crop was merely empiric and 

 possessed no value beyond the particular case to which 

 it referred. The science of agricultural chemistry may 

 be said to have been founded early in the nineteenth 

 century when Davy was appointed professor of chemical 

 agriculture to the Board of Agriculture. In the latter 

 part of the previous century, Priestley had shown that 

 green plants, when exposed to bright sunlight, decom- 

 pose the carbonic acid in the atmosphere into its elements 

 carbon and oxygen, keeping the carbon for themselves 

 and setting free the oxygen ; but this fact, and De 

 Saussure's work on plant chemistry, may be said to 

 represent the state of scientific knowledge of the subject 

 at the time. 



Davy did not make any very substantial contributions 

 to the science of agriculture, but he rendered valuable 

 service by insisting upon the value of studying agriculture 

 problems by scientific methods. He knew that the 

 farm and not the laboratory provided the final test of 

 the principles he expounded ; and he carried out some 

 field experiments himself. " Nothing is more wanting 

 in agriculture," he wrote, " than experiments in which 

 all the circumstances are minutely and scientifically 

 detailed. This art will advance with rapidity in pro- 

 portion as it becomes exact in its method." 



Twenty-five years after Davy's lectures, the great 

 French agricultural chemist, Boussingault, published 

 the results of detailed investigations of what may be 



