mt Subterranean Temperature. 48 



rapidly decomposed ; but for this certain circumstances are re- 

 quired, and these circumstances are always the production of 

 art, excepting in some natural cases, so rare and limited, that 

 they may be entirely kept out of sight. Before pyritic masses 

 of this kind are greatly altered, they require to be crumbled na- 

 turally, or reduced to fragments ; foe decomposition acts only in 

 the ratio of the surfaces. It is farther necessary that the cavi- 

 ties which inclose it, or that the rubbish in which it is contained, 

 be not too much or too little moistened, and that the circulation 

 of air be not too active, otherwise the alteration is very slow, 

 and then there is no sensible disengagement of heat. I shall 

 mention a remarkable example illustrative of this subject. 



The coal mines of St George Lavencas, in the department of 

 the Aveyron, consists of horizontal beds half a yard thick at 

 the most, and which are worked by galleries opening toward the 

 upper part of the dechvity, which borders to the west the im- 

 mense limestone platform of Larzac in which they are situ- 

 ated. The roof and floor of each bed are formed of a bitumi- 

 nous and pyritous schist, which was extensively worked, when 

 the price of copperas and' alum was much higher than at pre- 

 sent. The slate was allowed in a great measure to effloresce in 

 the mine before it was extracted. I visited these mines 

 long ago, and remarked no extraordinary elevation of tem- 

 perature in them. I returned on the 5th November 1822. 

 The works extended very far into the body of the moun- 

 tain, and their dryness was remarkable ; the galleries, ac- 

 cording to the custom of the country, were only high enough 

 to allow the miner to work on his side, and drag out the car- 

 riage loaded with the combustible by creeping. They were very 

 tortuous and frequently contracted. The air circulated very im- 

 perfectly, and in a very imperceptible manner. The pyritous 

 slate was every where in decomposition, whether at the surface 

 of the excavations, or amongst the numerous heaps of rubbish. 

 Yet the temperature of the air in the works nowhere exceeded 

 68°7. Now, it is to be remarked, that the external air was then 

 at 67°65 and that I experimented between two and three in the 

 noon. 



In general it is the case, that the circumstances adapted to 

 produce any remarkable degree of heat, by. the decomposition 



