22 HOW CROPS GROW. 



nutritive soil. These grand principles, understood in many 

 of their details, are an inestimable boon to agriculture, 

 and intelligent farmers have not been slow to apply them 

 in practice. The vast trade in phosphatic and Peruvian 

 guano, and in nitrate of soda; the great manufactures of 

 oil of vitriol, of superphosphate of lime, of fish fertilizers ; 

 and the mining of fossil bones and of potash salts, are 

 largely or entirely industries based upon and controlled 

 by chemistry in the service of agriculture. 



Every day is now the witness of new advances. The 

 means of investigation, which, in the hands of the scien 

 tific experimenter, have created within the writer s mem 

 ory such arts as photography and electro-metallurgy, and 

 have produced the steam engine and magnetic telegraph, 

 are working and shall continue to work progress in agri 

 culture. This improvement will not consist so much in 

 any remarkable discoveries that shall enable us &quot;to grow 

 two blades of grass where but one grew before,&quot; but in 

 the gradual disclosure of the reasons of that which we 

 have long known, or believed we knew, in the clear sepa 

 ration of the true from the seemingly true, and in the ex 

 change of a wearying uncertainty for settled and positive 

 knowledge. 



It is the boast of some who affect to glory in the suf 

 ficiency of practice and decry theory, that the former is 

 based upon experience, which is the only safe guide. But 

 this is a one-sided view of the matter. Theory is also 

 based upon experience, if it be truly scientific. The vaga- 

 rizing of an ignorant and undisciplined mind is not theory. 

 Theory, in the good and proper sense, is always a deduc 

 tion from facts, the best deduction of which the stock of 

 facts in onr possession admits. It is the interpretation of 

 facts. It is the expression of the ideas which facts awaken 

 when submitted to a fertile imagination and well-balanced 

 judgment. A scientific theory is intended for the nearest 

 poss ble approach to the truth. Theory is confessedly im- 



