240 



On the Application of Manures. 



which assists in the nutrition of plants, and increases their size. 

 This opinion is embraced without even an attempt being made 

 to discover the component parts of manure, or to become ac- 

 quainted with its nature." 



Whilst an eminent chemist expresses himself in these terms 

 with respect to the labours of physiologists, the judgment passed 

 upon the researches of chemists by a distinguished botanist of 

 our own country is not more favourable, since he quotes with 

 approbation a remark which had been m.ade to him by a friend, 

 to the effect that chemistry has hardly advanced the art of agri- 

 culture a single step, but that the latter remains, after all the in- 

 vestigations of the chemists, a mere empirical art." * 



I trust what I have said in my former lecture will convince 

 you that both these general statements must be received with 

 some grains of allowance, and that both chemistry and vegetable 

 physiology have already rendered good service to agriculture ; 

 but it cannot be denied that experience and tradition are still the 

 main sources to which the farmer usually looks for information, 

 and that science is as yet far from having shed any steady light 

 over the obscurity in which his processes are veiled. 



It is evident, that in order to determine in what precise manner 

 the different kinds of manure can improve the condition and in- 

 crease the quantity of the plants exposed to their influence, it 

 will be necessary in the first place to ascertain from what source 

 or sources vegetable substances obtain their nourishment ; and it 

 is certainly somewhat humiliating to reflect, that notwithstanding 

 the attention paid to the elucidation of this subject by many dis- 

 tinguished men of science, a question so fundamental should have 

 remained in part unanswered, until the celebrated German che- 

 mist, Liebig, applied himself to its investigation. 



The elements of which the structure of every perfect plant 

 consists may be referred to two distinct classes : namely, such as 

 are capable of being received into the vegetable organisation in 

 the condition of a gas, and such as are not. 



The former class of elements comprehends oxygen and hydro- 

 gen, which, either separately or in the condition of water, enter 

 into the constitution of every plant ; carbon, which, though of a fixed 

 nature when alone, is converted into a gaseous state when com- 

 bined either with oxygen or with hydrogen ; and nitrogen, which 

 is volatile both when alone and when in combination with hydro- 

 gen, as ammonia. 



The three former are universally present in plants, the latter 

 more commonly so than was formerly supposed, although in 



* Lindley's Introduction to Botany. 



