94 HOW CROPS FEED. = 
halation by sea-shore plants. It is found in the air near soda-works, be- 
ing a product of the manufacture, and is destructive to vegetation. 
Sulphurous Acid, 80,, and Sulphydric Acid, H8, (see 
H. C. G., p. 115,) may exist in the atmosphere as local emanations. In 
laree quantities, as when escaping from smelting works, roasting heaps, 
or manufactories, they often prove destructive to vegetation. In contact 
with air they quickly suffer oxidation to sulphuric acid, which, dissoly- 
ing in the water of rains, ete., becomes incorporated with the soil. 
Organic Matters of whatever sort that escape as vapor into the 
atmospliere and are there recognized hy their odor, are rapidly oxidized 
and have no dircet influence upon vegetation, so far as is now known. 
Suspended Solid Matters in the Atmosphere.— 
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-plants) 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 1862 (Jour. d'Ag. 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 ATMOSPIERIC SUPPLIES OF 
FOOD TO CROPS. 
Oxygen, whether required in the free state to effect 
chemical changes in the processes of organization, or in 
combination (in carbonic acid) to hecome 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, 
as is abundantly demonstrated by the experiments in wa- 
ter culture, made by Nobbe and Siegert, and by Wolff, 
(H. C. G., p. 170), in which oat and buckwheat plants 
were brought to more than the best agricultural develop- 
