ATMOSPHERIC AIR AS THE FOOD OF PLANTS. 33 
was not passing. The outer ends of the tubes ¢ and u were closed with 
caoutchouc tubes and glass plugs. 
In these experiments it was considered advisable to furnish to the 
plants more carbonic acid than the air contains. This was accomplished 
by pouring hydrochloric acid from time to time into the bottle 7, which 
contained fragments of marble. The carbonic acid gas thus liberated 
joined, and was swept on by the current of airin C. Experiments taught 
how much hydrochloric acid to add and how often, The proportion of 
this gas was kept within the limits which previous experimenters had! 
found permissible, and was not allowed to exceed 4.0 per cent, nor to 
fall below 0.2 per cent. 
In these experiments the seeds were deposited in a soil purified from 
nitrogen-compounds, by calcination in a current of air and subsequent 
washing with pure water. To this soil was added about 0.5 per cent of 
the ash of the plant which was to growin it, The water used for wa- 
tering the plants was specially purified from ammonia and nitric acid. 
The experiments of Lawes, Gilbert, and Pugh, fully 
confirmed those of Boussingault. For the numerous de- 
tails and the full discussion of collateral points bearing on 
the study of this question, we must refer to their elaborate 
memoir, “On the Sources of the Nitrogen of Vegetation.” 
—(Philosophical Transactions, 1861, II, pp. 431-579.) 
Nitrogen Gas is not Emitted by Living Plants.—It 
was long supposed by vegetable physiologists that when the foliage of 
plants is exposed to the sun, free nitrogen is evolved by them in small 
quantity. In fact, when plants are placed in the circumstances which 
admit of collecting the gases that exhale from them under the action of 
Might, it is found that besides oxygen a quantity of gas appears, which, 
unless special precautions are observed, consists chiefly of nitrogen, 
which was a part of the air that fills the intercellular spaces of the plant, 
or was dissolved in the water, in which, for the purposes of experiment, 
the plant is immersed. 
If, as Boussingault has recently (1863) done, this air be removed from 
the plant and water, or rather. if its quantity be accurately determined 
and deducted from that obtained in the experiment, the result is that no 
nitrogen gas remains. A small quantity of gas besides oxygen was indeed 
usually evolved from the plant when submerged in water. The gas on 
examination proved to be marsh gas. 
Cloéz was unable to find marsh gas in the air exhaled from either 
aquatic or land plants submerged in water, and in his most recent 
researches (1865) Boussingault found none in the gases given off from 
the foliage of a living tree examined without submergence. 
The ancient conclusion of Saussure, Daubeny, Draper, and others, 
that nitrogen is emitted from the substance of the plant, is thus showp 
to have been based on an inaccurate method of investigation. 
2 
