94 HOW CROPS FEED. 



h;>Lation hy sea-shore pLints. It is found in the air near soda-works, be- 

 ing a firoduct of the manufacture, and is destructive to vegetation. 



Niilpliiirou!i» Acid, SOo, and Siilpliydric Acid, HS, (see 

 H. C. G., p. 115,) may exist in the atmosphere as local emanations. In 

 lars^e quantities, as wlien escaping from smelting worlis, roasting lioaps, 

 or manufactories, they often prove destructive to vegetation. In contact 

 witli air tlicy qniclvly suffer oxid.ation to sulpiiurie acid, wliich, dissolv- 

 ing in the water of rains, etc., becomes incorporated with the soil. 



Oi'g^iiiiic Iflattei-s of whatever sort that escape as vapor into the 

 atmosphere and are tliere recognized I)y their odor, are rapidly oxidized 

 and have no direct influenee upon vegetation, so far as is now Icnown. 



Suspended Solid IWLiitters iit tlie Alinowpliei'e. — 

 The solid matters which are raised into the air by winds in the form of 

 dust, and are often transported to great heights and distances, do not 

 properly belong to the atmosphere, but to the soil. Their presence in 

 the air explains the growth of certain plants {air-planU) when entirely 

 disconnected from the soil, or of such as are found in pure sand or on 

 the surface of rocks, incapable of performing the functions of the soil, 

 except as dust accumulates upon them. 



Barral announced in 1863 {Jour. cTAg. pratique, p. 150) the discovery 

 of phosphoric acid in rain-water. Robinet and Luca obtained the same 

 result with water gathered near the surface of the earth. The latter 

 found, however, that rain, collected at a height of 60 or more feet above 

 the ground, was free from it. 



10. 



RECAPITULATION OF THE ATMOSPHERIC SUPPLIES OF 

 . FOOD TO CROPS. 



Oxyfi^en, whether required \n the free state to effect 

 chemical changes in the processes of organization, or in 

 combination (in carbonic acid) to become an ingredient 

 of the plant, is superabundantly supplied by the atmos- 

 phere. 



Carbon. — The carbonic acid of the atmosphere is a 

 source of this element sufficient for the most rapid growth, 

 ns is abundantly demonstrated by the experiments in wa- 

 ter culture, made by Nobbe and Siegert, and by Wolff, 

 (II. C. G., p. 170), in which oat and buckwheat plants 

 were brought to more than the best agricultural develop)- 



