364 



PRINCIPLES OF CHEMISTRY 



frequently occurs in coal-mines. 32 When mixed with air it forms an 

 explosive mixture, which forms one of the great dangers of coal mining, 

 AS subterranean work has always to be carried on by lamp-light. This 

 danger is, however, overcome by the use of Humphry Davy's safety 

 lamp. 33 Sir Humphry Davy observed that on introducing a piece of wire 

 .gauze into a flame, it absorbs so much heat that combustion does not 

 proceed beyond it (the unburnt gases which pass through it may be 

 ignited on the other side). In accordance with this, the flame of the 

 Davy lamp is surrounded with a thick glass (as shown in the drawing), 

 and has no communication whatever with the explosive mixture except 

 through a wire gauze which prevents it igniting the mixture of the 

 marsh-gas issuing from the coal with air. In some districts, particularly 

 in those where petroleum is found as, for instance, near Baku, where 

 a temple of the Indian fire- worshippers was built, and in Pennsylvania, 

 and other places marsh gas in abundance issues from the earth, and 

 it is used, like coal gas, for the purposes of lighting and warming. 34 



In practice illuminating gas is not only used for lighting (electricity and kerosene are 

 cheaper in Russia), but also as the motive power for gas engines (see p. 175), which con- 

 sume about half a cubic metre per horse-power per hour ; gas is also used in laboratories 

 for heating purposes. When it is necessary to concentrate the heat, either the ordinary 

 blowpipe (fig. 59) is applied, placing the end in the flame and blowing through the mouth- 

 piece ; or, in other forms, gas is passed through the blowpipe ; when a large, hot, smoke- 

 less flame is required for heating crucibles or glass-blowing, a foot-blower is used. High 

 temperatures, which are often required for laboratory and manufacturing purposes, are 

 most easily attained by the use of gaseous fuel (illuminating gas, producer gas, and 

 water gas, which will be treated of in the following chapter), because complete combus- 

 tion may be effected without an access of air. It is 

 evident that in order to obtain high temperatures 

 means must be taken to dimmish the loss of heat 

 by radiation, and to ensure perfect combustion. 



52 The gas which is set free in coal mines 

 contains a good deal of nitrogen, some carbonic 

 anhydride, and a large quantity of marsh gas. 

 The best means of avoiding an explosion consists 

 in efficient ventilation. It is best to light coal 

 mines with electric lamps. 



53 The Davy lamp, of which an improved form 

 is represented in the accompanying figure, is used 

 for lighting coal and other mines where combus- 

 tible gas is found. The wick of the lamp is en- 

 closed in a thick glass cylinder which is firmly 

 held in a metallic holder. Over this a metallic 

 cylinder and the wire gauze are placed. The pro- 

 ducts of combustion pass through the gauze, an<} 

 the air enters through the space between the 

 cylinder and the wire gauze. To ensure greater 

 safety the lamp cannot be opened without extin- 

 guishing the flame. 



84 In Pennsylvania (beyond the Alleghany mountains) many of the shafts sunk for 

 petroleum only emitted gas, but many useful Applications for it were found and it was 



FlG. 60. Davy safety-lamp. 

 [Modern form.] 



