TERRESTRIAL PHENOMENA. . 157 



ready spoken, possessed by that portion of physical science 

 whose origin is familiar to us, and is connected with our earth- 

 ly existence. The physical description of celestial bodies, from 

 the remotely-glimmering nebulae with their suns, to the central 

 body of' our own system, is limited, as we have seen, to gen- 

 eral conceptions of the volume and quantity of matter. No 

 manifestation of vital activity is there presented to our senses. 

 It is only from analogies, frequently from purely ideal com- 

 binations, that we hazard conjectures on the specific elements 

 of matter, or on their various modifications in the different 

 planetary bodies. But the physical knowledge of the het- 

 erogeneous nature of matter, its chemical differences, the reg- 

 ular farms in which its molecules combine together, whether 

 in crystals or granules; its relations to the deflected or de- 

 composed .waves of light by which it is penetrated ; to radi- 

 ating, transmitted, or polarized heat ; and to the brilliant or 

 invisible, but not, on that account, less active phenomena of 

 electro-magnetism — all this inexhaustible treasure, 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 ob- 

 jects 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 per- 

 mitted 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. 



Ii$ on the one hand, it were necessary to indicate the dif- 

 ference 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 labors of 

 roan; in the bores and shafts formed by miners. These labors* 



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

 man labor, we must recollect that there is a difference between the ah' 

 solute depth (that is to say, the depth below the Earth's 8Mrf**ce at th;it 



