22 CONSTITUENT MATERIAI * OF THE EARTH, 



amongst which coal takes a conspicuous place. The fa- 

 miliarly-known metals, as iron, tin, lead, silver, gold, an 

 elements of comparatively small magnitude in that exte- 

 rior part of the earth's body which we are able to inves- 

 tigate. 



It is remarkable of the simple substances that they are 

 generally in some compound form. Thus, oxygen and 

 nitrogen, though in union they form the aerial envelope 

 of the globe, are never found separate in nature. Carbon 

 is pure only in the diamond. And the metallic bases of 

 the earths, though the chemist can disengage them, may 

 well be supposed unlikely to remain long uncombined, 

 seeing that contact with moisture makes them burn. Com- 

 bination and re-combination are principles largely pervad- 

 ing nature. There are few rocks, for example, that are 

 not composed of at least two varieties of matter, each of 

 which is again a compound of elementary substances. 

 What is still more wonderful with respect to this princi- 

 ple of combination, all the elementary substances observe 

 certain mathematical proportions in their unions. One 

 Volume of them unites with one, two, three, or more vo- 

 lumes of another, any extra quantity being sure to be left 

 over, if such there should be. It is hence supposed that 

 matter is composed of infinitely minute particles or atoms, 

 each of which belonging to any one substance, can only 

 (through the operation of some as yet hidden law) asso- 

 ciate with a certain number of the atoms of any other. 

 There are also strange predilections amonst substances for 

 each other's company. One will remain combined in so- 

 lution with another, till a third is added, when it will aban- 

 don the former and attach itself to the latter. A fourth be- 

 ing added, the third will perhaps leave the first and join 

 the new comer. 



Such is an outline of the information which chemistry 

 gives us regarding the constituent materials of our globe. 

 How infinitely is the knowledge increased in interest 

 when we consider the probability of such being the mate- 

 rials of the whole of the bodies of space, and the laws un- 

 der which these everywhere combine, subject only to lo- 

 cal and accidental variations. 



In considering the cosmogonic arrangements of our 

 globe, ou* attention is called in a special degree to the 

 moon. 



In the nebular hypothesis, satellites are considered &j 



