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 

 capacious glass bottle or bell jar. The phosphorus 

 burns at first with a vivid light, which is presently ob- 

 scured by a cloud of snow-like phosphoric acid. The 

 combustion goes on, however, until nearly all the oxygen 

 is removed 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 volume 

 as its oxygen is removed, so that it is needful to pour 

 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. 



