180 — Bibliography. 
sists merely in its furnishing a slow and lasting source of carbonic 
acid, during its decomposition, which is absorbed by their roots, and 
constitutes the principal aliment of young plants, at a time when, be- 
ing destitute of leaves, they are unable to extract food from the atmos- 
phere. 
The existence of ammonia as a constant constituent of the atmos- 
here, had never been proved, or even suspected, before the re- 
searches of Prof. Liebig, and the great importance of the discovery, 
in a practical point of view, can be justly appreciated only by a careful 
perusal of the present treatise. In what manner ammonia is produced 
in quantity sufficient to be the chief, indeed the only means of convey- 
ing to plants all the nitrogen they contain, is fully elucidated. It is 
shown that rain water and snow always contain ammonia, and it may 
be proved to the satisfaction of any person, by adding a little sulphuric 
or muriatic acid to rain water, and evaporating it in a clean porcelain 
capsule nearly to dryness, when the ammonia may be detected by add- 
ing to the residuum a little powdered lime, which will liberate the am- 
monia. Thus produced, it always has an offensive animal odor, fully 
indicating its origin. Itis a most interesting thing, that in the discov- 
ery of ammonia in the atmosphere we have also discovered the true 
cause of the great fertilizing effects of gypsum, or plaster of Paris, 
a key to which has been so long sought in vain. s : 
This fertility arises exclusively from the fact that the sul of 
lime fixes in the soil the ammonia dissolved in the atmospher 
would otherwise be volatilized with the water as it evaporates. he 
carbonate of ammonia contained in rain water is decomposed by gyP” 
sum, in precisely the same manner as in the manufacture of sal am- 
moniac. Soluble sulphate of ammonia and carbonate of lime are form- 
ed, and this salt of ammonia possessing no volatility, is consequenly 
retained in the soil. The action of gypsum, or chloride calcium, 
(muriate of lime,) really consists in giving a fixed condition to the 
nitrogen or ammonia which is brought into the soil, and which 1s 10° 
dispensable to the nutrition of plants. The decomposition of gypsum 
y carbonate of ammonia, does not take place, however, instantane 
ously; on the contrary, it proceeds very gradually, and this explains 
why the action of gypsum lasts for several years. The reas? why 
the fact that ammonia is always present in the atmosphere has hereto- 
fore escaped observation, is, that the quantity in any portion of at 
mospheric air which is usually employed for analysis, is so &%¢ : 
ingly small that it might most naturally be overlooked, oF classed 
among the errors of observation. But the detection of ammonia must 
be much more easy when a pound of rain water is examined, which com” 
tains all the gas diffused through 20,800 cubic feet of air. If a poun 
iia. 
