AND OF THE OTHER BODIES OF SPACE. 19 



chlorine, are permanently gaseous j bromine is fluid at com- 

 mon temperatures ; and the remainder (with the exception of 

 fluorine, which has never been isolated, and whose physical 

 characters are consequently unknown) are solid. 



The body oxygen is considered as by far the most abundant 

 substance in our globe. It constitutes a fifth part of our atmo- 

 sphere, eight-ninths of the weight of water, and a large pro- 

 portion of every kind of rock in the crust of the earth. 

 Hydrogen, which forms the remaining part of water, and 

 enters into some minerals, is perhaps next. Nitrogen, of 

 which the atmosphere is four-fifths composed, must be consi- 

 dered as an abundant substance. The metal silicium, which 

 unites with oxygen in nearly equal parts to form silica, the 

 basis of about a half of the rocks in the earth's crust, is, of 

 course, an important ingredient. Aluminium, the metallic 

 basis of alumina, a material which enters largely into many 

 rocks, is another abundant elementary substance. So, also, is 

 carbon, a small ingredient in the atmosphere, 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 exterior part of the earth's body 

 which we are able to investigate. 



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 un- 

 likely 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 unchanging laws, of the 

 greatest beauty and simplicity. It has hence been surmised 



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