THE EARTH. 67 



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 indus- 

 try. The richest metals are very often less glit- 

 tering and splendid than the most useless mar- 

 casites, and the basest ores are in general 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 below its surface. The 

 combination of the different minerals with each 

 other, the heats which arise from their mixture, 

 the vapours they diffuse, the fires which they 

 generate, or the colds which they sometimes pro- 

 duce, are all either noxious or salutary to man ; 

 so that, in this great elaboratory of nature, a 

 thousand benefits and calamities are forging, of 

 which we are wholly unconscious j and it is hap- 

 py 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 pasing 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 continue working.* 



* Boyle, vol. iii. p. 233* 

 * 



