66 HISTORY OF 



longs to the history of fossils. It will be sufficient to observe 

 in this place, that as we descend into the mines, the various lay- 

 ers of earth are seen as we have already described them ; and in 

 some of these are always found the metals or minerals for which 

 the mine has been dug. Thus frequently gold is found dispers- 

 ed and mixed with clay and gravel ;' sometimes it is mingled 

 with other metallic bodies, stones, or bitumens ;^ and sometimes 

 united with that most obstinate of all substances, platina, from 

 which scarce any art can separate it. Silver is sometimes found 

 quite pure,' sometimes mixed with other substances and minerals. 

 Copper is found in beds mixed with various substances, marbles, 

 sulphurs, and pyrites. Tin, the ore of which is heavier than 

 that of any other metal, is generally found mixed with every 

 kind of matter -.* lead is also equally common ; and iron, we well 

 know, can be extracted from all the substances upon earth. 



The variety of substances which are thus found in the bowels 

 of the earth, in their native state, have a very different appearance 

 from what they are afterwards taught to assume by human in- 

 dustry. The richest metals are very often less glittering and 

 splendid than the most useless marcasites ; and the basest ores 

 are generally the most beautiful to the eye. 



This variety of substances, which compose the internal parts 

 of our globe, is productive of equal varieties, both above and be- 

 low its surface. The combination of the different minerals with 

 each other, the heats which arise from their mixture, the va- 

 pours they diffuse, the fires which they generate, or the colds 

 which they sometimes produce, are all either noxious or salutary 

 to man ; so that in this great elaboratoiy of nature, a thousand 

 benefits and calamities are forging, of which we are wholly un- 

 conscious ; and it is happy for us that we are so. 



Upon our descent into mines of considerable depth, the cold 

 seems to increase from the mouth as we descend ;^ but after 

 passing very low down, we begin by degrees to come into a 

 warmer air, which sensibly grows hotter as we go deeper, till at 

 last, the labourers can scarcely bear any covering as they ecu- 

 tintie working. 



This difference in the air was supposed by Boyle to proceed 



I I'll. 3, vol ii. p no 2 Ulkia, ibid. 3 Macquor's Chymistry, vol. i, p. rjl& 

 4 Hill's Fos:ils, p C.'S. 5 Eoyle, vol. iii. p. 231 



