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 
meang 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 “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 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 our 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 
possible approach to the truth. Theory is confessedly im- 
