22 HOW CROPS GROW. 



mainly that of diluting and tempering the affinities of 

 oxygen. Indirectly, however, it serves other most im- 

 portant uses, as will presently be seen. 



For the preparation of nitrogen we have only to remove 

 the oxygen from a portion of atmospheric air. This may 

 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 ox- 

 ygen a product that may be readily removed from the air 

 in which it is formed, the preparation of nitrogen from 

 ordinary 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 oxy- 

 gen is removed from the included air. The air is at 

 first expa'nded by the heat of the flame, and a portion 

 of it escapes from the vessel; afterward it diminishes 

 in volume as its oxygen is removed, so that it is need- 

 fill 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 nitro- 

 gen quite clear. 



EXP. 9. Another instructive method of preparing nitrogen is the fol- 

 lowing: A handful of green vitriol (protosulphate of iron or ferrous 

 sulphate, is dissolved in half a pint of water, the solution is put into 

 a quart bottle, a gill of ammonia-water or fresh potash-lye is added, 

 the bottle stoppered, 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 <.r nioie. until no further absorption takes place, when nearly 

 pure nitrogen remains in the bottle. 



Free nitrogen, under ordinary circumstances, mani- 

 fests no active properties, but is best characterized by its 

 chemical indifference to most other bodies. That it is 



