96 LESSONS m CHEMISTRT. 



cultivation they have been made to store up in their seeds 

 that large amount of gluten and other flesh-forming principles 

 for which they are at present so highly valued. 



1 37. The main object of the farmer is to improve and deve- 

 lope the wild plants of the field, or to keep those which in the 

 course of cultivation have been increased in value in proper 

 condition ; for experience has taught him that if he neglect 

 his duty they gradually deteriorate, and become again worth- 

 less for food. It is evident that he cannot properly understand 

 his business or expect to assist its progress without such 

 knowledge of the materials of his fields, and of the influence 

 which the air, the water, and the soil, exercise upon vege- 

 tables, as I have in the precedmg chapters endeavoured to 

 communicate. 



138. At a very early period, farmers were led to perceive, 

 that a close connexion must exist between the soil and the 

 kinds of plants produced upon it, that the wild plants of the 

 upland were different from those of the marsh, and that 

 every district had its peculiar flowers and grasses. It was 

 only, however, when chemistry had come to their assistance, 

 that the true nature of the connexion became intelligible. 

 We now know that certain plants flourish in some situations, 

 and refuse to gi'ow in other localities, because in the former 

 the soil is capable of yielding the materials which are requi- 

 site for their full development, while in the latter it is 

 incapable of supplying them with these materials. 



139. Nature of Exhaustion. — Experience has also taught 

 the farmer that though a field may be fertile, that is, fm-nished 

 with the earths, alkalies, and other materials essential to the 

 development of plants, yet that if the same crops be grown 

 upon it for a great number of years in succession, it will 

 sooner or later become incapable of yielding remunerating 

 harvests, unless assisted by art. 



Some useful information respecting the effect which the 

 growth of crops is capable of producing upon the soil may be 

 obtained by considering the practice of farmers in other coun- 

 tries. When the Irish emigrant in America bums down the 

 trees of the ancient forest in his new settlement, and commits 

 the seed to the freshly broken-up ground, he is astonished by 

 a produce far exceeding anything to which he had been ac- 

 customed in his former experience, and for years he obtains 

 immense crops of rich gi-ain without the necessity of manurmg 



