CONSTITUTION OF AIR 



119 



56. CHANGES WHICH TAKE PLACE DURING BURNING. 



It has been seen that there is a portion of the air, and only a 

 portion, which allows of the burning of a taper in it. It is important 

 that the nature of the changes which take place, both with regard to 

 the substance burnt and the air in which it burns, should be thoroughly 

 Understood. For this purpose we must use a substance which is 

 easily ignited. Phosphorus is a suitable substance, as it takes fire at a 

 temperature of 30 C. As this is below body heat, it is very dangerous 

 to handle it with the fingers. 



CAUTION. Phosphorus must never be taken in the hand, it must be cut 

 under water and removed by means of a tongs. 



i. White powder produced when phosphorus burns. Carefully dry a 

 wide glass cylinder and a small crucible. Cut off (under water) a piece of 

 phosphorus about half as big as a pea, and dry it between blotting paper 

 or filter paper. Using a pair of tongs, place the phosphorus in the crucible, 

 touch it with a hot wire, and quickly put the cylinder over it, as in Fig. 90. 



FIG. 90. Arrangement for collect- 

 ing the white powder produced by 

 burning phosphorus. 



FIG. 91. Method of opening ' 

 hole " plug, under water. 



side 



A white powder is deposited upon the sides of the cylinder. When the 

 phosphorus has ceased to burn, lift up the cylinder and pour a little water 

 into it. The white powder dissolves with a slight hissing noise. Add a 

 little blue litmus to the water ; the colour is changed to red. 



Note that the powder is deliquescent. 



ii. Proportion of air removed by burning 1 . Take a long test-tube, and 

 in it place a piece of phosphorus which has been carefully dried with filter 

 paper. Close with a one-hole rubber stopper, fitted with a piece of glass 

 tubing sealed at one end and having a small hole blown in one side (Fig. 91). 



