100 COMBUSTIBLE ACIDS. 



which are very much employed in chemical ana- 

 lyses, and with which, therefore, every chemist 

 ought to be familiarly acquainted ; and there 

 are some others which, though scarcely ever em- 

 ployed in chemical investigations, yet have ac- 

 quired considerable importance in consequence 

 of their connection with the physiology of vege- 

 tables, or animals. I shall, therefore, in this 

 chapter treat of the atomic weights of nine dif- 

 ferent combustible acids. 



SECT. I. 



OF THE ATOMIC WEIGHT OF OXALIC ACID. 



THIS is, upon the whole, the most important of 

 all the combustible acids as far as chemical ana- 

 lysis is concerned. It is usually crystallized, 

 and the apparent figure is a flat four-sided prism 

 terminated by a bihedral summit; but Mr. 

 Brooke informs us that the primary form is an 

 Crystalline oblique rhombic prism. The rhombic base, to 



shape of ox- 

 alic acid, the unexperienced eye, passes for one of the 



sides of the flat prism ; and as the crystal is 

 usually attached by one of its sides, two of the 

 lateral faces appear to the eye as the dihedral 

 summit. Two of the opposite lateral edges are 

 usually deeply truncated, which makes the prism 

 six-sided instead of four-sided.* These crystals 



* Annals of Philosophy, (second series) VI. 119. 

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