588 
FOOD OF PLANTS. 
that name ; consisting of vegetable matters, partly decom¬ 
posed, but not completely disorganized ; resembling a plant 
"without organization, pervading and nourishing others. 
Humus being slightly soluble in water, it was supposed to 
yield materials capable of being absorbed by the roots of 
plants, and the soluble parts were called “ humic acid/’ and 
the insoluble “ humin,” or coal of humus; carbon being, as 
usual, the chief constituent element. But it has been found 
that humus requires 2500 times its own weight of water to 
dissolve it—a much greater quantity than nature affords ; 
and the very trifling portion of alkalies in many soils would be 
insufficient to yield any great supply of carbon from that 
source. On lands that are regularly mown, humus increases, 
and yields as much carbon as manured grounds. It is thence 
conjectured that the air supplies the carbon, as it has been 
incontestably proved that plants imbibe carbonic-acid gas, 
retain the carbon, and emit the oxygen. Water is a prime 
agent, and the elements of it may be decomposed and as¬ 
similated by plants at the same time. From the very small 
quantity of carbonic-acid gas in the air, and of carbon with 
the gas itself, it may be supposed that the vegetative process 
possesses the power both of assimilation and of augmentation. 
Experiments made by supplying carbonic acid and water to 
plants failed, because other necessary ingredients were 
wanting. The decaying vegetable matter, by converting the 
oxygen into carbonic acid, affords the first food to plants till 
the leaves are able to perform the functions of inhalation. 
The carbon is thus derived from two sources ; from the humus 
in the soil in the first place, and then from the atmosphere. 
Ammonia, in different forms, is a powerful promoter of 
vegetation, and is thought to afford the nitrogen to plants, 
being itself a compound of nitrogen and hydrogen ; or, 
ammonia 17=14 nitrogen x 3 hydrogen. Ammonia exists 
largely in manures of various kinds, and in plants it forms 
several colours; it abounds in the last products of all animal 
substances, and in rain and in snow-water; the smell exhaled 
by the ground after the melting of snow most sensibly showing 
its presence. The use of gypsum, charcoal, and burned 
clay arises from their fixing and retaining ammonia ; and 
oxides and ferruginous matters also possess that property. 
It has been supposed that manures act only by the formation 
of ammonia; but nitrogen, or the product of ammonia, 
exists sparingly in vegetables, and with the nature of that 
elementary body, whether it be simple or compound, we are, 
as yet, very imperfectly acquainted. Plants contain a great 
preponderance of carbon over the azote, and animal bodies the 
