THEORIES OF ITS JUSE. 161 



agriculture has been gradually founded, the prin- 

 cipal object of which is to obtain the greatest 

 possible produce with the least expense of manure. 



Now it was deduced from all the foregoing facts 

 that plants require for their growth different con- 

 stituents of soil, and it was very soon perceived, 

 that an alternation of the plants cultivated main- 

 tained the fertility of a soil quite as well as leaving 

 it at rest or fallow. It was evident that all plants 

 must give back to the soil in which they grow 

 different proportions of certain substances, which 

 are capable of being used as food by a succeeding 

 generation. 



But agriculture has hitherto never sought aid 

 from chemical principles, based on the knowledge 

 of those substances which plants extract from the 

 soil on which they grow, and of those restored to 

 the soil by means of manure. The discovery of 

 such principles will be the task of a future genera- 

 tion, for what can be expected from the present, 

 which recoils with seeming distrust and aversion 

 from all the means of assistance offered it by 

 chemistry, and which does not understand the 

 art of making a rational application of chemical 

 discoveries ? A future generation, however, will 

 derive incalculable advantage from these means of 

 help. 



Of all the views which have been adopted regard- 

 ing the cause of the favourable effects of the alter- 

 nations of crops, that proposed by M. Decandolle 



M 



