BY THE PERCEPTION OP SUCH EELATIONS. 37 



And in the soil a skilful analyst might often pass over the 

 minute proportions of potash, or of phosphoric acid, contained 

 in it, were he not assured, by their presence in the plant, that 

 he ought to find traces of them in the soil on which it grows. 



The composition of the animal, also, is rectified and improved 

 by similar reasoning in another direction. The soil and the 

 plant, for example, both abound in silica, but analysis detected 

 it in the parts of the animal body, in traces so minute, that 

 they could not be regarded as necessary in any respect to its 

 healthy growth. But in the feathers, the covering of birds, 

 it was found to exist always in very considerable proportion ; 

 and hence, the hair, the covering of animals, being more 

 minutely examined, silica was recognised to be always present 

 in, and apparently necessary to it, in small proportion. Thus 

 it ought also to be necessarily present in the blood, and in the 

 other fluids, on its way to or from the hair, and farther research 

 has shown this to be the case. 



Thus our knowledge is in its nature progressive. Analyses 

 of one kind or of one substance lead to rectifications in analyses 

 of another kind or of another substance. It is not by improved 

 methods alone that our analytical results are made more perfect. 

 Our eyes are sharpened when we learn what to look for, and 

 with the same methods and skill we shall discover substances 

 we had not previously attempted to detect. Not only does the 

 composition of the animal tell us what must be in the plant, and 

 in the soil, but, as the example above given shows, the recog- 

 nised use and necessary presence of a substance in an animal of 

 one class, may lead to researches which, contrary to received 

 opinions, may show it to be necessary to animals of other classes 

 also. 



So it is with experiments in the field and the feeding-house. 

 Those which I am about to suggest in the following pages will 

 rectify past results and suggest new researches. The results of 

 these, again, will send us back to revise our opinions and repeat 

 our analyses ; and thus, by the joint aid of the laboratory, the 

 field, and the feeding-house, will scientific agriculture be carried 

 slowly but steadily forward. 



