> € 
- |  ~— Bibliography. 193 
standard of our present belief on this subject, that we are hencefor- 
ward to consider humus as playing a much less important part in the 
“nutrition of vegetables than has heretofore been attributed to it. We 
are to consider it rather as a slow and inexhaustible source of car- 
bonic acid, which is continually given out by it in its various stages 
of deeay until it attains the condition of perfect mould ; still it can- 
not be divested of a certain degree of importance, since, as a general 
tule, it is a usual constituent of fertile soils. But the instances addu- 
ced by Prof. Liebig as io its solubility in water, even in its most so- 
luble form, the crenate of lime, go far to prove, that water alone is 
never the means of conveying it into the animal organism. Yet it 
must ever be allowed to bea proof of discernment and analytical 
skill on the part of Dr. Jackson, to have proved so conclusively as 
his numerous experiments seem to do, the identity of the principal 
mass of the substance called humus with crenic and apocrenic acids. 
The methods followed by Berzelius, in his analysis of the ochre 
from Porla water, were found inadmissible in the case of soils, for 
caustic potassa takes up a considerable portion of alumina and silica, 
and decomposes several saline combinations occurring in soils. The 
alkaline carbonates are decomposed by the crenic and apocrenic acids, 
the alkali combining with them. Carbonate of potash was accordingly 
at first employed, in accordance with the recommendation of Dr. Da- 
ha; but it cannot by washing be entirely removed from the soil; and 
the subearbonate takes up a portion of the alumina. Carbonate of 
ammonia was accordingly tried, and with perfectly good success, for 
it can be entirely washed from the soil, and takes up all the organic 
matter which can be supposed to be useful in vegetation, offering there- 
fore the best means for comparative experiments. In practice, a giv- 
€n weight of the pulverized and dry soil to be examined, is placed in 
a glass flask and covered with a solution of carbonate of ammonia in 
pure water, saturated at 60°; this is then placed in a situation where 
the temperature is about 170°, or the solution may be gently boiled. 
When it is judged that the carb. ammonia has taken up all it can, 
turn off the dark brown solution upon double filters previously coun- 
lerpoised. Repeat the digestion with the ammoniacal carbonate as 
long as the soil imparts toitany color. After washing well the filters 
and drying them with the insoluble matter in the drying closet, coun- 
terpoise the filters against each other and ascertain the difference of 
Weight; the loss is soluble organic matter taken up by the ammonia. 
€ soil may then be burned in a platina crucible, and the loss is in- 
Soluble vegetable matter. Acidulate now the ammoniacal solution 
With acetic acid, drop in gradually a solution of acetate of copper. 
The brown floceu t precipitate which falls is apocrenate of copper. 
840. 20 
Vol. xz, No. 1.—Oct.—Dec. 1 
