ELEMENTARY SUBSTANCES. 



33 



specific gravity in all stones or earthy minerals if it much exceed 

 2*5, or approach 3 may be attributed : in other words, if they are 

 nearly three times heavier than an equal bulk of water. Gems and 

 the earths barytes and strontian are exceptions; but these never form 

 entire rocks. The presence of iron not only increases the weight, 

 and darkens the colour of numerous rocks and stones, but is one 

 principal means of their decomposition, for iron exists in stones in 

 two states of oxygenation, as the black or the red oxide ; and when 

 the former is exposed to air and moisture, it absorbs a greater por- 

 tion of oxygen, and is converted into a brown ochrey incrustation, 

 which peels off, and exposes a fresh surface of the stone to a similar 

 process. 



Manganese, in a state of oxide, occurs in a few rocks, to which it 

 generally communicates a dull reddish colour inclining to purple, and 

 a peculiarly dry and burnt-like appearance. 



Sulphur, though found in considerable masses, cannot, by itself, 

 be regarded as a constituent part of rocks ; but when it is combined 

 with oxygen, forming sulphuric acid, it unites with hme, and forms 

 the well-known mineral gypsum or plaster stone. 



Carbon, or Charcoal, enters, as a constituent part, into many of 

 the slate rocks, to which, it generally, communicates a dark colour : 

 it forms also regular beds of considerable thickness, being the princi- 

 pal constituent part of coal. Carbon, combined with oxygen, forms 

 carbonic acid or fixed air, which is combined and solidified in all 

 limestone rocks, in a proportion exceeding two fifihs of the whole 

 weight. As carbon exists in such a large proportion even in the old- 

 est hmestones, we may regard it as a constituent element, and not 

 as a substance derived from the vegetable kingdom. For whence 

 did the vegetables themselves derive their carbon ? 



Potass and Soda. — These alkalies occur in minerals which com- 

 pose parts both of primary and volcanic rocks ; but the proportion 

 is so small, that they would scarcely deserve the attention of the ge- 

 ologist, did not the latter alkali, soda, exist in such abundance in the 

 waters of the ocean and in rock salt. Pure sea salt, or rock salt, 

 contains nearly 53J parts of soda, 46 J muriatic acid or chlorine. 



Muriatic acid, combined with soda, is the only state in which this 

 acid forms a constituent part of any rocks we are yet acquainted 

 with ; except in some volcanic rocks, where it may be regarded as 

 accidental. 



Phosphoric Acid, combined with calcareous earth, is a principal 

 constituent of animal bones : it occurs, also, in a few limestone beds, 

 which are supposed to have derived phosphoric acid from the decom- 

 position of animal matter. This acid is of very rare occurrence in 

 the mineral kingdom. 



The above elementary substances, either separately or combined, 

 form all the simple minerals of which rocks are composed. A knowl- 

 edge of these minerals, and their different intermixtures and combi- 



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