20 CONSTITUENT MATERIALS OF THE EARTH 



earth's crust, is, of course, an important ingredient. Alumi- 

 nium, the metallic basis of alumina, a material which enters 

 largely into many rocks, is another abundant elementary sub- 

 stance. So, also, is carbon, a small ingredient in the atmo- 

 sphere, but the chief constituent of animal and vegetable 

 substances, and of all fossils which ever were in the latter 

 condition, amongst which coal takes a conspicuous place. 

 The familiarly -known metals, as iron, tin, lead, silver, gold, 

 are elements of comparatively small magnitude in that ex- 

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

 tigate. 



It is remarkable of the elementary substances that they 

 generally exist in combination. Thus, oxygen and nitrogen, 

 though in mixture 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. Combination and re- combination 

 are principles largely pervading 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 principle of combination, all the elementary substances 

 observe certain mathematical proportions in their unions. 

 When in the gaseous state, one volume of them unites with 

 one, two, three, or more volumes of another, any extra 

 quantity being sure to be left over, if such there should be. 

 Combinations by weight are also governed by fixed and un- 

 changing laws, of the greatest beauty and simplicity. It has 

 hence been surmised that matter is composed of infinitely 

 minute particles or atoms, each of which belonging to any 

 one substance can only associate with a certain number of the 

 atoms of any other. There are also strange predilections 

 amongst substances for each other's company. One will re- 

 main combined in solution with another, till a third is added, 

 when it will abandon the former and attach itself to the 



