346 



SCIENTIFIC RECREATIONS. 



In the case of a common candle, or lamp, combustion takes place in 

 the same way. The wick is the intermediary. The oil mounts in the lamp 

 wick, where it is converted into a gas by heat ; it then "takes fire," and 

 gives us light and heat. The candle-flame is just the same with one excep- 

 tion : the burning material is solid, not liquid, though the difference is only 

 apparent, for the wax is melted and goes up as gas. The burning part 

 of the wick has a centre where there is no combustion, and contains carbon. 

 We can prove this by placing a bent tube, as in the illustration (fig. 326), 

 one end in the unburning part of the flame. We shall soon see a dark vapour 



Fig. 329. Davy's safety lamp. 



Fig. 330. Davy lamp (section). 



come over into the receiver. This is combustible, for if we raise- the tube 

 without the glass we can light the gas (fig. 327). If we insert the end of 

 the tube into the brilliant portion of the flame we shall perceive a black 

 vapour, which will extinguish the combustion, for it is a mixture of carbonic 

 acid gas and aqueous vapour, in which (fig. 328) particles of carbon are 

 floating. 



When we proceed to light our lamps to read or to write by, we find 

 some difficulty in making the wick burn at first. We present to it a lighted 

 taper, and it has no immediate effect. Here we have oil and cotton, two 



