OF THE NITROGEN OF PLANTS. 87 



sequently retained in the soil. All the gypsum 

 gradually disappears, but its action upon the car- 

 bonate of ammonia continues as long as a trace of 

 it exists. 



The beneficial influence of gypsum and of many 

 other salts has been compared to that of aromatics, 

 which increase the activity of the human stomach 

 and intestines, and give a tone to the whole system. 

 But plants contain no nerves ; we know of no sub- 

 stance capable of exciting them to intoxication and 

 madness, or of lulling them to sleep and repose. 

 No substance can possibly cause their leaves to 

 appropriate a greater quantity of carbon from the 

 atmosphere, when the other constituents which the 

 seeds, roots, and leaves require for their growth are 

 wanting. The favourable action of small quantities 

 of aromatics upon man, when mixed with his food, is 

 undeniable, but aromatics are given to plants with- 

 out food to be digested, and still they flourish with 

 greater luxuriance. 



It is quite evident, therefore, that the common 

 view concerning the influence of certain salts upon 

 the growth of plants evinces only ignorance of its 

 cause. 



The action of gypsum or chloride of calcium really 

 consists in their giving a fixed condition to the 

 nitrogen or ammonia which is brought into the 

 soil, and which is indispensable for the nutrition of 

 plants. 



In order to form a conception of the effect of 



