THE FOOD OF THE PLANT 37 



the same as the residual gas left after the burning of 

 phosphorus in air. 



Inference. — Damp iron filings take up the active gas 

 from the air. 



The name of the active gas in the air is oxygen and 

 that of the inactive is nitrogen. 



There are, in the air, other gases in small quantities, 

 and these will be dealt with later. They were, of course, 

 present in the residual gas left after the phosphorus 

 had been burnt, since only the oxygen was taken from 

 the air. It must therefore be remembered that the 

 nitrogen tested was impure nitrogen, although the 

 quantity of other gases contained in it was small. It is 

 possible to prepare pure nitrogen, but it will not be 

 very easy for us to do so at this stage. A pure specimen 

 of the gas would give the same observations except in 

 the case of the lime-water, which would not be turned 

 milky at all, thus showing that the slight milkiness 

 formed in the lime-water was due to some impurity 

 and not to the nitrogen itself. 



We have already tested the properties of nitrogen 

 and found it to be an inert gas in which a taper will 

 not burn. The properties of oxygen must next be 

 tested. In order to do this a specimen of the gas must 

 be prepared. 



When phosphorus was burnt in air it united with the 

 oxygen, forming a white powder called an oxide of 

 phosphorus which rapidly dissolved in the water. It is 

 extremely difficult to get the oxygen back again from 

 the oxide of phosphorus, but many other substances 

 containing oxygen give it up quite easily. Such a one 

 is potassium chlorate. It is a white, crystalline sub- 

 stance, and is used in the making of fireworks and 

 matches. When potassium chlorate is heated, oxygen 

 is given off. It is given off even more readily and 

 with less heat if a black substance named manganese- 

 dioxide is mixed with it. The manganese-dioxide 

 remains unchanged at the end of the reaction, but in 

 some way it helps the potassium chlorate to decompose. 



