Al-aiOSPIIERlC AIU AS THE FOOD OF PLANTS. 06 



was not passing. The outer ends of the tubes t and u were closed with 

 caoutchouc tubes and glass plugs. 



In these experiments it was considered advisable to furnish to the 

 plants lUDre carbonic acid than the air contains. This was accomplished 

 by pouring hj-drochloric acid from time to time into the bottle T, which 

 contained fragments of marble. The carbonic acid gas thus liberated 

 joined, and was swept on by the current of air in C. E.Kperiments taught 

 how much hydrochloric acid to add and how often. Tlie proportion of 

 this gas was kept within tlie limits which previous experimenters had 

 found permissible, and was not allowed to exceed 4.0 per cent, nor to 

 fall below 0.^ per cent. 



In these experiments the seeds were deposited in a soil [uirilied 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 wliieh was to grow in it. Tlie water used for wa- 

 tering the plants was specially purified from ammonia and nitric acid. 



The experiments of Lawes, Gilbert, and Pngh, fully 

 confirmed those of Boussmgaiilt. 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 Xitrogen of Vegetation." 

 . — {Philosophical Transactions, 1831, II, pp. 431-579.) 



I\'iti-ogeiL Oils is not I^iiiitte«l l>y I.iiving' I'liiiits. — It 

 was long supposed by vegetable physiologists that wlien the foliage of 

 plants is exposed to the sun, free nitrogen is evolved by them in small 

 quantitj'. In fact, when plants are placed in the circumstances which 

 admit of coll-^cting the gases tliat exhale fi'om them under the action of 

 light, it is found tliat besides oxj-gen a quantity of gas appears, which, 

 unless special precautions arc observed, consists chieily of nitrogen, 

 which was a iiart of tlie 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 fpQm 

 the plant and water, or rather if its quantity be'accurately determined 

 and deducted from that obtained in the experiment, the result is tliat 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. 



Cloez was unaljle to find marsh gas in tlie air exhaled from either 

 aquatic or hind plants submerged in water, and in his most recent 

 researches (186.5) Boussingault found none in the gases given tiflf from 

 the foliaire of a living tree examined without submergence. 



The ancient conclusion of Saussure, Daubeuy, Draper, and others, 

 that nitrogen is emitted from tlie substance of the plant, is thus showo 

 to have been based on an inaccurate method of investigation. 



2* 



