COMBUSTION OF A CANDLE. 145 



present, it will be seen that about i of the gaseous contents of the 

 bottle have been absorbed, but that the odd |- have remained un- 

 absorbed, being the nitrogen. 



Expt. 149. To show that Nitrogen extinguishes a Candle. 

 The reason why combustible bodies, such as candles, burn in the 

 air is simply that chemical changes go on between the burning 

 material and the oxygen of the atmosphere; if the oxygen is 

 removed, the residual nitrogen will no longer keep up the chemical 

 change, and the flame is consequently extinguished. Lower a 

 lighted taper at the end of a wire, or a spill of wood or paper, into 

 the bottle containing nitrogen obtained in the last experiment, and 

 you will see that the flame is extinguished, just as it was by 

 carbon dioxide in Expt. 71; and for the same reason in each case, 

 viz., that there is no oxygen present to keep the flame alight. 



For other methods of preparing nitrogen, see Expts. 183 and 184. 



Expt. 150. To show that a fresh Supply of Air is requisite 

 to keep up the Combustion of a Candle. Fix a lighted candle 

 at the end of a wire, and dip it into a tall narrow cylinder (fig. 40), 

 and cover the mouth of the cylinder with a plate. In a short 

 time the candle will go out, the flame becoming extinguished, 

 because all the available oxygen in the cylinder has become used 

 up by chemical action. 



Now withdraw the candle, relight it, and place it in a second 

 similar jar, and slip into the jar a piece of wet cardboard previously 

 cut to size, so as to form a vertical partition in the middle of the 

 upper part of the jar. If properly placed in position, the candle 

 will now continue to burn, because a ventilating current is set up 

 in the jar, the hot products of combustion rising up on one side 

 of the pasteboard partition, and fresh air descending down the 

 other side, so as to keep the candle supplied with air. That the 

 two currents are set up in opposite directions is easily shown by 

 lighting a piece of brown paper, and blowing out the flame, so 

 that the paper smoulders and gives off a thick smoke ; bring the 

 smoking end of the paper torch to one side of the partition, and 

 the smoke will be carried downwards towards the candle by the 

 descending current of air; whilst on the other side the smoke 

 will be carried upwards by the ascending current of hot gases and 

 vapours from the flame. 



The pasteboard is wetted to prevent its possibly taking fire at 

 the candle flame ; a strip of metal (sheet copper, zinc, or tinplate, 

 &c.) may be conveniently used instead. 



This system of ventilation by means of a shaft divided by a 

 " brattice " or vertical partition is sometimes used in mines, a large 

 fire being employed to produce the up-current. 



K 



