Di* Davy's Agricultural Discourse. 2&6 



temperature and other influences oil which its increase and 

 diminution depend. And, besides these ingredients, there is 

 reason to infer, that various other substances are either sus- 

 pended, floating as minute particles or dissolved in the air, 

 such as carbonate of ammonia, common salt, and some other 

 salts derived from the sea, and dust of several kinds. The 

 most important of all these atmospheric ingredients are, un- 

 doubtedly, the oxygen, azote, carbonic acid, aqueous vapour, 

 and ammoniacal salt. I should exceed my limits as to time, 

 were I to enter into particulars on the fertilizing agency of 

 either one of these substances. Aqueous vapour condensed 

 and precipitated, you know, occasions rain, — is, in fact, rain. 

 When it falls, it brings down with it carbonic acid, some oxy- 

 gen and azote, and, there is reason to believe, a minute portion 

 of carbonate of ammonia — all dissolved in it. Moistening dead 

 vegetable and animal matter at the surface of the earth, it 

 favours their decomposition, and the evolution of carbonic 

 acid and of carbonate of ammonia ; penetrating beneath the 

 surface, descending into the soil, it has a like eff'ect there ; 

 and there becoming impregnated by its solvent j^ower with 

 what is found in a fertile soil, — the substances already al- 

 luded to, as phosphate of lime, carbonate of limd and mag- 

 nesia, carbonate of potash, and silica, — it passes into the 

 growing plant, absorbed by its roots, and becomes its nutri- 

 tive sap. Thus complicated is rain ih its agency ; so various 

 are the circumstances which concur to this agency, and so 

 happily are they connected, one favouring the action of the 

 other, and all promoting the process of vegetation. Reflect- 

 ing on these circumstances, we cease to wonder at the growth 

 of forests in a state of nature, in which for a long series of 

 years, vegetable matter, living or dead, is constantly accu- 

 mulating, deriving its elements solely from the atmosphere 

 and the soil, and by what it abstracts from the former greatly 

 enriching the latter. 



2dli/. Of the fertilizing means derived from animal mattet* 

 I have already alluded to the composition of animal matter, 

 and how, as regards its ultimate elements, excepting that it 

 contains no silica, it diff"ers, compared with those of vege- 

 tables, rather in proportion than kind. Readily putrefying 



