6 HOW CROPS GROW. 



know. They did not know that the atmosphere dissolves 

 the rocks, and converts inert stone into nutritive soil. 

 These grand principles, understood in many of their de- 

 tails, are an inestimable boon to agriculture, and intelli- 

 gent farmers have not been slow to apply them in prac- 

 tice. 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 indus- 

 tries largely or entirely 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, the telegraph, the tele- 

 phone and the electric light, are working and shall ever- 

 more continue to work progress in the art of agriculture. 

 This improvement will not consist so much in any re- 

 markable discoveries that shall enable us to ' grow two 

 blades of grass where but one grew before;" but in the 

 gradual disclosure of the reasons of that which we have 

 long known, or believed we knew; in the clear separa- 

 tion 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 worth the name. The 

 fancies of an ignorant and undisciplined mind are not 

 theory as that term is properly understood. Theory, in 

 the strict scientific sense, is always a deduction from 

 facts, and the best deduction of which the stock of facts 

 in our possession admits. It is therefore also the inter- 



