36 HOW ANALYSES ARE CORRECTED 



does not refer in its strictness to general farm experiments for 

 private use, and with a view to economical ends only. Such 

 experiments neither require the same preparation on the part 

 of the experimenter, nor the same care and scrupulous accu- 

 racy in conducting the experiments. General results are often 

 enough for such purposes, and a failure may be sufficient 

 to deter, or apparent success to urge on, the practical man, 

 though neither the amount nor the cause of either may be 

 clearly understood. 



9. How analyses are corrected through the perception of such 

 relations^ and how we arrive at an exact knowledge of the com- 

 position of the plant, the soil, or the animal. 



I conclude this chapter by observing that the perception of 

 such relations as are explained in the preceding section is not a 

 mere intellectual gratification, or a help only to the practical 

 man in his experiments it directly promotes the progress of 

 knowledge, suggests new experimental researches, and points 

 out new methods of approaching the truth. Thus, in regard to 

 our actual knowledge of the composition, absolute and com- 

 parative, of soils, plants, and animals, we have by no means been 

 able to arrive at it, directly and at once. It is by the employ- 

 ment of two very different methods that it has been attained, 



First, by direct analysis of the soil, the plant, or the animal. 



Second, by reasoning backward to the plant from the ascer- 

 tained composition of the animal that feeds upon it, and to the 

 soil from the composition both of the animal and of the plant. 



Thus, in the plant, we are led to detect substances, and to 

 determine their quantity, the essential or necessary nature of 

 which to the plant itself we deduce only from the fact of their 

 being always present in the animal in notable proportion. Such 

 is the case with oxide of iron, with oxide of manganese, and with 

 fluorine. The proportion of one or other of these substances 

 which exists in a plant, is frequently so small that it would en- 

 tirely escape detection, were we not urged to more close exa- 

 mination by a knowledge of its existence in the animal ; or if it 

 were detected, we should, without such knowledge of the animal, 

 be inclined to consider it as only accidentally present. 



