96 rroAv <nor^ vv.v.r). 



prove that it consists of oxygen. For this purpose bring 

 tlie water outside the neck to u level M'ith that inside; 

 have ready a splinter of pine, the end of which is glow^ 

 ing hot, but not in flame, remove the cork, and insert the 

 ignited stick into the gas. It will inflame and burn much 

 more brightly than in the extei-nal air. (See H, C. G., p. 

 35, Exp. 5.) To this phenomenon, one of the most ini« 

 portant connected with our subject, we shall recur under 

 the head of carbonic acid, the compound which is the 

 chief source of this exhaled oxygen. 



RELATIONS OF NITROGEN GAS TO VEGETABLE NUTRITION. 



Nitrogen Gas not a Food to the Plant. — Nitrog n in 

 the free state appears to be indifferent to vegetation. 

 Priestley, to wliom we are much indebted for our knowl- 

 edge of the atmosphere, was led to believe in 1779 that 

 free nitrogen is absorbed by and feeds the plant. But 

 this philosopher had no adequate means of investigating 

 the subject. De Saussure, twenty yeais later, having 

 command of better methods of analyzing gaseous mix- 

 tiires, concluded from his experiments that free nitrogen 

 does not at all j^articipate in vegetable nutrition. 



BouSSingault's Experiments. — The question rested un- 

 til 1837, when Boussingault made some trials, which, how- 

 ever, were not decisive. In 1851-1855 this ingenious 

 chemist resumed the study of the subject and conducted 

 a large number of experiments with the greatest care, 

 all of which lead to the conclusion that no appreciable 

 amount of free nitrogen is assimilated by plants. 



His plan of experiment was simply to cause plants to 

 grow in circumstances where, every other condition of de- 

 velopment being supplied, the only source of nitrogen at 



