74 HISTORY OF 



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 difference, as yet, 

 remains a secret. 



As from this account of mines it appears that 

 the internal parts of the globe are filled with va- 

 pours of various kinds, it is not surprising that 

 they should at different times reach the surface, 

 and there put on various appearances. In fact, 

 much of the salubrity, and much of the unwhole- 

 someness 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 ; and over others the fields are 

 found destitute of verdure.t The inhabitants, 

 also, are rendered dreadfully sensible of these 

 subterraneous exhalations, being affected with 

 such a variety of evils proceeding entirely from 

 this cause, that books have been professedly writ- 

 ten upon this class of disorders. 



* PhiL Trans. vol.ii. p. 523. f Boyle, vol. iii. p. 238. 



