92 . HOW CROPS FEED. 
The mode of ifs origin at once suggests its presence in the atmos- 
phere. Saussure observed that common air contains some gaseous com- 
pound or compounds of carbon, besides carbonic acid; and Boussin- 
gault found iu 1834 that the air at Paris contained a very small quantity 
(from two to cight-millionths) of hydrogen in some form of combina- 
tion besides water. These facts agree with the supposition that marsh 
gas isa normal though minute and variable ingredient of the atmosphere, 
Relations of Marsh Gas to Vegetation.—Whether 
this gas is absorbed and assimilated by plants isa point on which we 
have at present no information. It might serve as a source both of car- 
bon and hydrogen; but as these bodies are amply furnished by carbonic 
acid and water, and as it is by no means improbable that marsh gas it- 
self is actually converted into these substances by ozone, the question 
of its assimilation is one of little importance, aud remains to be inves- 
tigated. 
Schultz (Johnston’s Lectures on Ag. Chem., 2d Ed., 147) found on sev- 
eral occasions that the gas evolved from plants when exposed to the sun- 
light, instead of being pure oxygen, contained a combustible admixture, 
so that it exploded violently on contact with a lighted taper. 
This observation shows either that the healthy plants evolved a large 
amount of marsh gas, which forms with oxygen an explosive mixture 
(the fire-damp of coal-mines), or, as is most probable, that the vegetable 
matter entered into decomposition from too long continuance of the 
experiment. 
Boussingault has, however, recently found a minute proportion of 
marsh gas in the air exhaled from the leaves of plants that are exposed 
to sunlight when submerged in water. It does not appear when the leaves 
are surrounded by air, as the latest experiments of Boussingault, Cloez, 
and Corenwinder, agree in demonstrating. 
Carbonic Oxide, CO, isa gas destitute of color and odor. It 
burns in contact with air, with a flame that has a fine blue color, The 
result of its combustion is carbonic acid, CO + O = CO,. 
This gas is extremely poisonous to animals. Air containing a few 
per cent of it is unfit for respiration, and produces headache, insensi- 
bility, and death. 
Carbonic oxide may be obtained artificially by a variety of processes, 
If carbonic acid gas be made to stream slowly through a tube containing 
ignited charcoal, it is converted into carbonic oxide, CO, + C = 2C0. 
Carbonic oxide is largely produced in all ordinary fires. The air which 
draws through a grate heaped with well-ignited coals, as it enters the 
bottom of the mass of fuel, loses a large portion of its oxygen, which 
there unites with carbon, forming carbonic acid. This gas is carried up 
into the heated coal, and there, where carbon is in excess, it takes up an- 
other proportion of this element, being converted into carbonic oxide. 
At the summit of the fire, where oxygen is abundant, the carbonic oxide 
burns again with its peculiar blue color, to carbonic acid, provided the 
heat be intense enough to inflame the gas, as is the case when the masa. 
