38 HOW CROPS GROW. 
be accomplished more or less perfectly by a variety of 
methods. We have just learned that the process of burn- 
ing is a chemical union of oxygen with the combustible. 
If, now, we can find a body which is very combustible and 
one which at the same time yields by union with oxygen 
a product that may be readily removed from the air in 
which it is formed, the preparation of nitrogen from ordi- 
nary air becomes easy. Such a body is phosphorus, a 
substance to be noticed in some detail presently. 
Exp. 8.—The bottom of a dinner-plate is covered half an inch deep 
with water, a bit of chalk hollowed out into a little cup is floated on the 
water by means of a large flat cork or a piece of wood; into this cup a 
morsel of dry phosphorus as large as a pepper-corn is 
placed, which is then set on fire and covered by a 
ecapacious glass bottle or bell jar. The phosphorus 
burns at first with a vivid light, which is presently ob- 
secured by a cloud of snow-like phosphoric acid. The 
combustion goes on, however, until nearly all the oxygen 
is remaved from the included air. The air is at first ex- 
panded by the heat of the flame, and a portion of it es- = 
capes from the vessel; afterward it diminishes in yolume Fic. 5 
as its oxygen is removed, so that it is needful to pour BONG, 
water on the plate to prevent the external air from passing into the 
vessel. After some time the white fume will entirely fall, and be absorbed 
by the water, leaving the inclosed nitrogen quite clear. 
Exp. 9.—Another instructive method of preparing nitrogen is the fol- 
lowing: A handful of copperas (sulphate of protoxide of iron) is dis- 
solved in half a pint of water, the solution is put into a quart bottle, a 
gill of liquid ammonia or fresh potash lye is added, the bottle stopper- 
ed, and the mixture vigorously agitated for some minutes; the stopper 
is then lifted, to allow fresh air to enter, and the whole is again agitated 
as before; this is repeated occasionally for half an hour or more, until 
no further absorption takes place, when nearly pure nitrogen remains in 
the bottle. 
Free nitrogen, under ordinary circumstances, has scarce- 
ly any active properties, but is best characterized by its 
chemical indifference to most other bodies. That it is in- 
capable of supporting combustion is proved by the first 
method we have instanced for its preparation. 
Exp. 10.—A burning splinter is immersed in the bottle containing the 
nitrogen prepared by the second method, Exp. 9; the flame immediately 
goes out. 
