PRINCIPLES OF THE SAFETY LAMP. 139 



the wire gauze alone which protects the miner 

 when he gropes his way, with lamp in hand, into 

 a part of the mine where fire-damp may have 

 collected. 



The common explanation of this is as follows : 

 The explosive mixture will not take fire unless 

 the ignited body applied to it is at a white heat. 

 The flame of the wick is, it is true, at a white 

 heat, and would therefore cause it to ignite 

 and explode immediately ; but before this flame 

 could pass to the fire-damp it must pass through 

 the wire gauze, and in so doing, it becomes very 

 much cooled by the conducting powers of the 

 metallic wire of which the gauze is made. The 

 consequence is, that it would be no longer at a 

 white heat, and that the fire-damp therefore 

 would not take light. The reason, then, why 

 the safety -lamp is a safe light, is that the cooling 

 properties of the wire gauze prevent the passage 

 of the flame at a sufficiently high temperature 



to set fire to the explosive gas. If the reader 



t . 

 will take a piece of wire gauze, and hold it over 



the flame of a candle, he will find that for a 

 little time the flame will not pass through, and 

 that he can, in fact, look down into the centre 

 of the flame, which is hollow (fig. 2). After a 

 time, however, the wire becomes so heated, that 

 the flame does pass through, and then presents 

 the appearance represented at fig. 1 in the cut. 



