34 EXPERIMENTAL PLANT PHYSIOLOGY 



burning of the candle in it, since it is no longer able 

 to support combustion. 



Again, a fire will not burn without a " draught " ; 

 that is, a constant supply of fresh air must be allowed 

 to pass through it. 



Both these observations denote an intimate connection 

 between the air and combustion; it is well, therefore, 

 to begin investigations by burning something and then 

 try to find out whether the air has been altered in any 

 way by the combustion of the substance in it. 



Phosphorus is commonly chosen for such investiga- 

 tion because it burns very readily. It is a yellow, waxy 

 substance, and is poisonous. The smell of burning 

 matches is due to the phosphorus contained in them. 



Phosphorus should on no account be touched with 

 the fingers, because the heat of the body is quite 

 suflScient to cause it to ignite. Indeed it must always 

 be kept under water, because it burns so readily and 

 violently in air. 



If a piece about the size of a pin's head is burnt in 

 a porcelain dish the bright yellow flame and the dense 

 white fumes may be noticed. 



EXPEEIMENT 17 



Aim. — To find out any facts about the composition of 

 air by burning phosphorus in it. 



Method. — In order to test any change that takes 

 place in the air due to the burning, it will be necessary 

 to enclose a portion of air. 



Float a little porcelain dish in a trough of water. 

 Remove a stick of phosphorus from the bottle with a 

 pair of forceps ; put it between blotting-paper and cut 

 off a piece about the size of a pea. Do not touch 

 the phosphorus with the fingers; replace the stick 

 immediately. 



Then put the piece of phosphorus into the dish and 

 coyer with a stoppered bell-jar. While the bell-jar is 

 being put into position the stopper must be taken out. 

 You will readily see why this is necessary. Replace the 



