GENERAL VIEW. 149 



themselves, whether crystalline or granular; its relations to 

 the deflected, or decomposed waves of light by which it is 

 penetrated; to radiating, and to transmitted, or polarised 

 heat; to the brilliant, or the not less energetic because 

 invisible, phenomena of electro-magnetism, all this in- 

 calculable treasure of physical knowledge by which our 

 contemplation of the universe is enriched and exalted, we 

 owe to investigations concerning the surface of the planet 

 which we inhabit, and more to its solid than to its liquid 

 portion. I have already noticed how greatly this extensive 

 knowledge of natural objects and forces, and the measure- 

 less variety of objective perceptions, stimulates the cultiva- 

 tion and promotes the activity of the human intellect ; it is 

 as needless, therefore, to dwell farther on this topic, as on 

 that of its connection with the causes of the superiority in 

 material power, which particular nations derive from their 

 command of a portion of the elements. If, on the one 

 hand, I have been desirous of calling attention to the dif- 

 ference between the nature of our telluric knowledge, and of 

 that which we possess concerning the regions of space, I 

 wish, on the other hand, to indicate the limited extent of 

 the field from whence our whole knowledge of the hetero- 

 geneous properties of matter is derived. It is from that 

 which has been rather inappropriately termed the " crust" 

 of the earth, or the thickness of so much of the strata 

 nearest to the surface of our planet, as is opened to our 

 view either by deep natural valleys, or by the labours of 

 man in boring or in mining operations. These opera- 

 tions ( 124 ) attain a perpendicular depth below the level of 

 the sea of little more than two thousand feet, about one-third 

 of a geographical mile, or -g-gVo of the Earth's radius. The 



