MANUKES AND THE PEINCIPLES 

 OF MANUEING. 



HISTOEICAL INTBODUCTIOJST. 



AGRICULTURAL CHEMISTRY, like most branches of nat- 

 ural science, may be said to be entirely of modern 

 growth. While it is true we have many old specu- 

 lations on the subject, they can scarcely be said to 

 possess much scientific value. The great questions 

 which had first to be solved by the agricultural 

 chemist were, What is the food of plants ? and, 

 What is the source of that food ? The second of 

 these two questions more easily admitted of answer 

 than the first. The source of plant-food could only 

 be the atmosphere or the soil. As the composition 

 of the atmosphere, however, was not discovered till 

 the close of last century, and the chemistry of the 

 soil is a question which is still requiring much work 



