THE EARTH. 57 



from m^azines of fire that lay nearer the centre, and that diifas- 

 ed 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 admission 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 observ- 

 able heat is perceived below, without the necessity of recurring 

 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 composed j 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 quan- 

 tity 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 ex- 

 periment, 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 operation. Those 

 beautiful minerals called marcasites and pyrites, are often of this 

 composition ; and wherever they are found, either by imbibing 

 the moisture of the air, or having been by any means combined 

 with water, they render the mine considerably hot.® 



The want of fresh air also, at these depths, is, as we have said, 

 another reason for their being found much hotter. Indeed, 

 without the assistance of art, the bottom of most mines would, 

 from this cause be insupportable. To remedy this inconve- 

 nience, the miners are often obliged to sink, at some convenient 

 distance from the mouth of the pit where they are at work, 

 another pit, which joins the former below, and which, in Derby- 

 shire, is called an air-shaft. Through this the air circulates ; 

 and thus the workmen are enabled to breathe freely at the bot- 

 tom of the place ; which becomes, as Mr Boyle affirms, very 

 commodious for respiration, and also very temperate as to heat 

 and cold.' Air Locke, however, who has left us an account of 



6 Klrcher Mund. Subt. vol. ii. p. 216. 7 Eoyle, vol iii. y. 23a 



