ATMOSPHERIC AIR AS THE FOOD OF PLANTS. 91 
The nitrates do not appear to be absorbed by the plant 
to any great extent, except through the medium of the 
soil, since they cannot exist in the state of vapor and are 
brought down to the earth’s surface by atmospheric waters. 
The full discussion of their nutritive effects must there- 
fore be deferred until the soil comes under notice. See 
Division II, p. 271. 
In § 10, p. 96, “Recapitulation of the Atmospheric 
Supplies of Food to Crops,” the inadequacy of the at- 
mospheric nitrates will be noticed. 
§ 9. 
OTHER INGREDIENTS OF THE ATMOSPHERE; viz., Marsh Gas, 
Carboiie Oxide, Nitrous Oxide, Hydrochloric Acid, Sulphurous Acid, 
Sulphydrie Acid, Organic Vupors, Suspended Solid Matters. 
There are several other gaseous bodies, some or all of which may oc- 
cur in the atmosphere in very minute quantities, but whose relations to 
vegetation, in the present state of our knowledge, appear to be of no 
prictical moment. Since, however, they have been the subjects of in- 
vestigations, or disquisition by agricultural chemists, they require to be: 
briefly noticed. 
Marsh Gas,* C H,.—This substance is a colorless and nearly. 
odorless gas, which is formed almost invariably when organic matters 
suffer decomposition in absence of oxygen. When a lump of coal ora 
billet of wood is strongly heated, portions of carbon and hydrogen. 
unite to form this among several other substances. It is accordingly 
one of the ingredients of the gases whose combustion forms the flame’ 
of all fires and lamps. It is also produced in the decay of vegetable mat- 
ters, especially when they are immersed in water, as happens in swamps 
and stagnant ponds, and it often bubbles in large quantities from the 
bottom of ditches, when the mud is stirred. 
Pettenkofer und Voit have lately found that marsh gas is one of the 
gascous products of the respiration or nutrition of animals. 
It is combustible at high temperatures, and burns with a yellowish, 
faintly luminous flame, to water and carbonic acid. It causes no ill ef- 
fects when breathed by animals if it be mixed with much air, though of 
itself it cannot support respiration, 
* Known also to chemists under the names of Light Carburetted Hydrogen, 
Hydride of Methy] and Methane. 
