26 HOW CROPS FEED. 
prove that it consists of oxygen. For this purpose bring 
the water outside the neck toa level with 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 external air. (See H. C. G., p. 
35, Exp. 5.) To this phenomenon, one of the most im. 
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. 
§ 3. 
RELATIONS OF NITROGEN GAS TO VEGETABLE NUTRITION. 
Nitrogen Gas not a Food to the Plant.—Nitrogn in 
the free state appears to be indifferent to vegetation. 
Priestley, to whom 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 years later, having 
command of better methods of analyzing gaseous mix- 
tures, concluded from his experiments that free nitrogen 
does not at all participate 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 
