THE SAFETY LAMP. 



347 



things which would speedily set a warehouse in flames from top to bottom, 

 but we cannot even ignite them, try all we can. Why ? Because we must 

 first obtain a gas, oil will not burn liquid ; it must be heated to a gaseous 

 point before it will burn, as all combustion depends upon that, so flames 

 mount high in air. Now in a candle-flame, as will be seen in the diagram 

 (fig. 329), there are threa portions, the inner dark core, which 

 consists of unburnt gas ; the outer flame, which gives light ; and 

 the outside rim of perfect combustion non-luminous. In the 

 centre, A, there is no heat. If we place a piece of gauze wire 

 over* the flame at a little distance the flame will not penetrate it. 

 It will remain underneath, because the wire, being of metal, 

 quickly absorbs the heat, and consequently there is no flame. 

 This idea led to the invention of the " safety " lamp by Sir 

 Humphrey Davy, which, although it is not infallible, is the 

 only lamp in general use in mines (figs. 329, 330) 



Mines must have light, but there is a gas in mines, a "marsh" 

 gas, which becomes very explosive when it mixes with oxygen. 

 Of course the gas will be harmless till it meets oxygen, but, in a candlt flarae> 

 its efforts to meet, it explodes the moment the union takes place ; instead 

 of burning slowly like a candle it goes off all at once. This gas, called 

 "fire damp," is carburetted hydrogen, and when it explodes it develops into 

 carbonic acid gas, which suffocates the miners. 



Fig. 332. Pouring carbonic acid on a 

 lighted taper. 



