6t HISTOUV OF 



Of this kind are the vapours of the mines about Bristol ; on 

 the contrary, in other mines, a single spark struck out from the 

 collision of flint and steel, would set the whole shaft in a flame. 

 In such, therefore, every precaution is used to avoid a collision ; 

 the workmen making use of wooden instruments in digging ; and 

 being cautious, before they enter the mine, to take out even the 

 nails from their shoes. Whence this strange difl!'erence should 

 arise, that the vapours of some mines catch fire with a spark, and 

 others only with a flame, is a question that we must be content 

 to leave in obscurity, till we know more of the nature both oi 

 mineral vapour and of fire. This we only may observe, that 

 gunpowder will readily fire with a spark, but not with the flame 

 of a candle ; on the other hand, spirits of wine will flame with a 

 candle, but not with a spark : but even here the cause of this 

 difl^erence as yet remains a secret. 



As from this account of mines, it appears that the internal 

 parts of the globe are filled with vapours of various kinds, it is 

 not surprising that they should, at different times, reach the sur- 

 face, and there put on various appearances. In fact, much of 

 the salubrity, and much of the unwholesomeness, of climates 

 and soils, is to be ascribed to these vapours, which make their 

 way from the bowels of the earth upwards, and refresh or taint 

 the air with their exhalations. Salt mines, being naturally cold,' 

 send forth a degree of coldness to the external air, to comfort 

 and refresh it : on the contrary, metallic mines are known not 

 only to warm it with their exhalations, but often to destroy all 

 kinds of vegetation by their volatile corrosive fumes. In some 

 mines, dense vapours are plainly perceived issuing from their 

 mouths, and sensibly warm to the touch. In some places neither 

 snow nor ice will continue on the ground that covers a mine ; 



himself of its absolute safety, in order that he might induce the miners of liis 

 parish to use it, half of whom he liad seen so lately swept away by the 

 dreadfu! explosion before narrated. 

 The invention of this lamp has produced a new era in the coal mining ol 

 ^ Grertt Britain. The steel mills were very expensive, and in certain cases 

 produced explosions, whereas the safety-lamp can be carried without dan- 

 ger amongst inflammable gas ready to explode : and although the wire be^ 

 come red-hot, an explosion of the gas will take place inside of the lamp, 

 without communicating inHammation to the external gas. 



I I'l.i!. Trans, vol. ii. p. 523. 



