ALUM MINES OF AUBIN. 355 



daction of a considerable quantity of aluminous salts amid 



the torrefied rocks. 



The burning pits, whence the alum works originate, The pits de- 



^must be considered as totally destroyed: but the alum pro- stroyed, but 



compensated 

 duced will more than compensate for the loss of the coal, by the alum. 



It is known tooy that the fire will go out of itself, as soon 

 as it has consumed all the masses of fuel, that have been ex- 

 posed by the levels. It has long been ascertained, that the The fire does 



fire does not extend more than a yard into ihe coal left un- not burn mora 



^ , , '' than a yard 



touched in depth. This is so certain, that the extraction of deep. 



the coal from beneath the works burned has been resumed at 

 Lassalle and Fontaines. 



The effects of the spontaneous combustion of the coal are Effects of the 

 the same in the four alum works. To judge from the stat0 co«ib«stioa. 

 of the surface of the earth, the fire has not extended beyond 

 the space that had been worked. The surface has sunk, 

 cracked, and been deranged, in the manner of volcanic sol- 

 faterras. It emits a gentle heat, incessantly renewed; it is 

 bestrewed with very curious produtrtioas of fusion and torre- 

 f^ction; the crevices emit burning fumes of sulphurous 

 acid, bitumen, and water; and even flames continually 

 arise when the fire is consuming a stratum near the surface. 

 The sandstones and schists, that accompany the burning 

 seams of coals, are either simply torrefied, or changed into 

 red, light, and rugged scorias, or violet-coloured, bluish, gray, 

 and often striped, enamels. The acidosulphurous vapours 

 attack, deprive of colour, and decompose part of these 

 products, and frequently reduce them to powder ; and at 

 their expense are formed the vitriolic saline substances, that 

 are found in such great abundance, either in the cavities of 

 the masses, or amid the earth resulting from their decompo- 

 sition, or on the surface of the ground. The simple or 

 alkaline* sulphate of alumine constitute almost the whole of 

 these saline substances. They exhibit themselves in all forms ; 

 in disseminated particles, discoverable only by their acerb and 

 styptic taste, in whitish efflorescences, and filamentous and' 



* The alkali is probably furnished either by the combustion of 

 the coal, or the decomposition of the feldspar; which abounds in 

 the rocks affected by the acidosulphurous vapours. 



A a 2 «ilk/ 



