I. AGRICULTURE AND 'RURAL ECONOMY. 407 



of carbon and nitrogen) in a continual, unchanging circula- 

 tion through these kingdoms, as water, alone, was at one 

 time thought capable of. It was also shortly after sought 

 for in animal fluids, especially in the blood ; and as soon as 

 improved processes, especially of analytical chemistry, ren- 

 dered it possible, the general statement became allowable 

 that j^hosphoric acid is present in all animal fluids, circulates 

 through all the channels of the body, and is absolutely indis- 

 pensable in building up the solid portions, and in the perform- 

 ance of the functions of the most important fluids, although 

 it was, in many cases, detected with difficulty in all the sol- 

 ids and fluids of animals, on account of its very small per- 

 centage. The origin of this phosphorus in the animal must 

 be sought for in the vegetable kingdom, and it may be as- 

 sumed that one per cent, of phosphoric acid is present in 

 grain, one third per cent, in straw, one half per cent, in air- 

 dried hay, one fifteenth per cent, in potatoes, and one twelfth 

 per cent, in undried fodder. However, although it was rec- 

 ognized as indispensable to vegetable nutriment, agricultur- 

 ists had no thought of supplying it or any other ingredient 

 to the soil, artificially, until the well-known chemist, Klap- 

 roth, in 1799, in his investigation of leucite, was led by the 

 unusual loss in his analyses to the discovery of potash in that 

 mineral. He saw in this discovery the possibility of an en- 

 tire revolution in the system of natural history ; and from 

 this point vegetable chemistry took a new departure, by ear- 

 nestly inquiring into the origin of the constituents of the 

 ashes, instead of remaining satisfied with the simple assump- 

 tion that they were produced by the plant. Hence the in- 

 valuable practical results of the search after the source of 

 phosphoric acid, and the final conclusion that such plants as 

 grow on soils apparently free from phosphorus, and which 

 yet accumulate phosphoric acid in the seed, are able to de- 

 tect this substance, so essential to their life, more readily 

 than chemical reagents can. The idea of maintaining and 

 assisting the proper life of a plant by furnishing the neces- 

 sary elements was first broached by Liebig, who showed that 

 plants must obtain materials for their growth from the soil, 

 instead of from the air and water alone, as previously sup- 

 posed. Waters were examined as to the substances which 

 they brought from the earth in solution, soils were analyzed, 



