NATURE AND SOURCES OF FOOD. 93 



over the heap. If- ammonia ~be present, copious white fumes 

 will be perceived, which result from the chemical union of the 

 hydrochloric acid gas with the ammoniacal gas, and the forma- 

 tion of a salt, the hydrochlorate of ammonia, or the sal 

 ammoniac of the stores. 



Although ammonia is constantly rising in vast quantities 

 into the atmosphere from decaying animal and vegetable 

 matter, it is nevertheless easily soluble in water, and is there- 

 fore prevented from accumulating there by the aqueous vapor 

 of the atmosphere, which, when it is precipitated thence in the 

 form of rain, conveys the ammonia in solution to the roots of 

 plants. That this is the fact is evident because ammonia can 

 be detected in rain water and in the sap of plants, and also 

 because all manures such as guano, which contain a great 

 amount of ammonia, are found to be fertilizing to soils. 



The combustible or organic part of the plant forrns by far 

 the greater part of its structure. This is evident from the 

 small amount of ash or inorganic matter left after its incinera- 

 tion. It follows that plants derive the materials of their 

 growth mainly from the atmosphere. 



That certain plants derive the greater part of their food 

 from the atmosphere, affords an explanation of the process by 

 which nature changes the barren rock into the fertile soil. 

 The first plants which clothe the surface of the newly formed 

 coral reef, or of our common rocks, are lichens and mosses ; 

 plants which derive the greater part, if not the whole of their 

 nutriment, entirely from the atmosphere. Now plants can only 

 grow in proportion to the quantities of food afforded them. 

 Lichens and mosses are plants of very humble growth and 

 exceedingly simple structure, consisting of, comparatively 

 speaking, only a few cells. Successive generations of these 



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