THE ELEMENTS OF PLANT FOOD 21 



can be done by causing it to oxidize some substance 

 with which it will readily combine. 



The usual method is to place a small piece of phos- 

 phorus on a large flat cork, float the cork on water, and 

 cover it with a jar. The phosphorus uniting readily 

 with the oxygen in the jar leaves the N with some fumes 

 of a compound of phosphorus and oxygen. These 

 fumes will soon be absorbed by the water, leaving the 

 N comparatively pure. 



Notes. As the oxygen is combined with phosphorus, water rises 

 and fills the space left vacant about one fifth of the jar. This shows 

 the volume of O to be about one fifth and N to be about four fifths 

 of the ah-. 



A piece of burning candle may be placed on the cork instead of the 

 phosphorus. The product of the combustion is CO 2 . If now the 

 mixture be collected over lime water, the CO a will be removed and 

 the N will remain nearly pure. 



Chemical Properties. --The chief characteristic of N 

 is its inertness. The difference between O and N in 

 this regard may be shown by placing a jar of O and a 

 jar of N near each other and rapidly passing a lighted 

 candle from one jar to the other. The N will put out 

 or diminish the flame, and the O will relight it from a 

 spark remaining on the wick. This may be repeated a 

 number of times. Although four fifths of the air we 

 take into our lungs is N, we get none of the N that 

 is in our bodies from this source. The N that we 

 obtain comes through the food that we eat. 



Plants are surrounded with air that contains the N, 

 but most plants appropriate little or none of the free 

 N. They obtain their supply of N through their roots 

 in the form of nitrates in solution. 



