68 HISTORY OF 



This difference in the air was supposed by 

 Boyle to proceed from magazines of fire that lay 

 nearer the centre, and that diffused their heat 

 to the adjacent regions. But we now know that 

 it may be ascribed to more obvious causes. In 

 some mines the composition of the earth all 

 around is of such a nature, that, upon the ad- 

 mission of water or air, it frequently becomes 

 hot, and often bursts out into eruptions. Besides 

 this, as the external air cannot readily reach the 

 bottom, or be renewed there, an observable heat 

 is perceived below, without the necessity of re- 

 curring to the central heat for an explanation. 



Hence, therefore, there are two principal causes 

 of the warmth at the bottom of mines : the heat 

 of the substances of which the sides are compos- 

 ed ; and the want of renovation in the air below. 

 Any sulphureous substance mixed with iron, 

 produces a very great heat, by the admission of 

 water. If, for instance, a quantity of sulphur be 

 mixed with a proportionable share of iron filings, 

 and both kneaded together into a soft paste, 

 with water, they will soon grow hot, and at last 

 produce a flame. This experiment, produced by 

 art, is very commonly effected within the bowels 

 of the earth by nature. Sulphurs and irons are 

 intimately blended together, and want only the 

 mixture of water or air to excite their heat ; and 

 this, when once raised, is communicated to all 

 bodies that lie within the sphere of their opera- 

 tion. Those beautiful minerals called marcasites 

 and pyrites, are often of this composition ; and 

 wherever they are found, either by imbibing the 



