148 COSMOS. 



V 



of matter. No manifestation of vital activity is there pre- 

 sented to our senses. It is only from analogies, frequently 

 from purely ideal combinations, that we hazard conjectures 

 on the specific elements of matter, or on their various modifi- 

 cations in the different planetary bodies. But the physical 

 knowledge of the heterogeneous nature of matter, its chemical 

 differences, the regular forms in which its molecules combine 

 together, whether in crystals or granules ; its relations to the 

 deflected or decomposed waves of light by which it is pene- 

 trated ; to radiating, transmitted, or polarised heat ; and to 

 the brilliant or invisible, but not on that account less active 

 phenomena of electro-magnetism all this inexhaustible trea- 

 sure, by which the enjoyment of the contemplation of nature 

 is so much heightened, is dependent on the surface of the 

 planet which we inhabit, and more on its solid than on its 

 liquid parts. I have already remarked how greatly the study 

 of natural objects and forces, and the infinite diversity of the 

 sources they open for our consideration, strengthen the mental 

 activity, and call into action every manifestation of intellectual 

 progress. These relations require, however, as little comment 

 as that concatenation of causes by which particular nations are 

 permitted to enjoy a superiority over others in the exercise of 

 a material power derived from their command of a portion of 

 these elementary forces of nature. 



If. on the one hand, it were necessary to indicate the diffe- 

 rence existing between the nature of our knowledge of the 

 Earth and of that of the celestial regions and their contents, I 

 am no less desirous on the other hand to draw attention to 

 the limited boundaries of that portion of space from which we 

 derive all our knowledge of the heterogeneous character of 

 matter. This has been somewhat inappropriately termed the 

 Earth's crust ; it includes the strata most contiguous to the 

 upper surface of our planet, and which have been laid open 

 before us by deep fissure-like valleys, or by the labours of man, 

 in the bores and shafts formed by miners. These labours* do 



* In speaking of the greatest depths within the Earth reached by- 

 human labour, we must recollect that there is a difference between the 

 absolute depth (that is to say, the depth below the Earth's surface at that 

 point) and the relative depth (or that beneath the level of the sea). The 

 greatest relative depth that man has hitherto reached is probably the bore 

 at the new salt works at Minden, in Prussia : in June, 1844, it was 



