USE OF GYPSUM. 53 



plants ; and this is quite sufficient for the purposes of agricuL 

 ture. Agriculture differs essentially from the cultivation of 

 forests, inasmuch as its principal object consists in the production 

 of the constituents of the blood ; whilst the object of forest 

 culture is confined principally to the production of carbon. But 

 the presence of ammonia alone does not suffice for the production 

 of the nitrogenous ingredients. Other conditions likewise are 

 quite essential. All the various means of culture are sub- 

 servient to these two main purposes. A part only of the car 

 bonate of ammonia conveyed by rain to the soil is received by 

 plants, because a certain quantity of it is volatilized with the 

 vapor of water ; only that portion of it can be assimilated which 

 sinks deeply into the soil, or which is conveyed directly to the 

 leaves by dew, or is absorbed from the air along with the 

 carbonic acid. 



Liquid animal excrements, such as the urine with which the 

 solid excrements are impregnated, contain only a small part of 

 their ammonia in the state of salts, that is, in a form in which it 

 has completely lost its volatility. The greatest part exists in the 

 form of carbonate of ammonia — a salt of great volatility. When 

 the ammonia is presented in the condition of a fixed salt, not the 

 smallest portion of it is lost to plants ; it is all dissolved by water, 

 and imbibed by their roots. The evident influence of gypsum 

 upon the growth of grasses — the striking fertility and luxuriance 

 of a meadow upon which it is strewed — depends, in some degree, 

 upon its fixing in the soil the ammonia of the atmosphere, which 

 would otherwise be volatilized, with the water which evaporates.* 



* I made the following experiment on a small garden plot. Beans and 

 peas were planted in the soil, after it had heen well manured hy mixing it 

 with fresh horse-dung. The whole surface of the plot was strewed with 

 gypsum to the depth of a line, and then covered so as to be protected from 

 the rain. In dry weather it was duly watered. 



The plants soon appeared above ground and flourished with great luxuri- 

 ance. Before the commencement of the experiment, I had examined both 

 the soil and the gypsum, and found that both were quite free from the 

 smallest trace of carbonates. But on testing some of the gypsum taken 

 from the surface after the lapse of several weeks, I ascertained that the 

 greatest part of it had been converted into carbonate of lime. All th« 

 soil to the depth of half a foot now effervesced strongly on the addition of 

 acid. 



