34 G. H. KNIBBS. 



H H 



I I 



C— C 



H— C< ^C— H 



x c— cr 



I I 



H H 



The reasoning by which in di-substitution products the ortho- or 

 vicinal, the meta- and para- positions are identified, is equally 

 ingenious. It is obvious from these simple illustrations that the 

 determination of structure often calls for the exercise both of a 

 comprehensive knowledge of chemistry and also of genius; and the 

 enormous number of organic substances, the structure of which 

 has been ascertained beyond all doubt, is a testimony not only to 

 the assiduity but also to the splendid intuition of the world's 

 chemists. 



Organic substances fall into two great divisions, viz., the open- 

 chain or aliphatic series, and ring-compounds or the aromatic series ; 

 the latter may also have monovalent radicles from the former 

 externally attached. One of the most significant achievements 

 of modern chemistry has been the formation of ring-compounds 

 from open-chain ones ; as when, on heating mannoheptitol with 

 hydriodic acid, it is reduced to tetrahydrotoluene. Another achieve- 

 ment of equal potentiality is the converse process of breaking up 

 ring-compounds into open-chain ones ; as in the chlorination of 

 phloroglucinol, which converts it into dichloracetic acid and tetra- 

 chloracetone ; or in its hydrolysis through heating with strong 

 caustic alkali, which breaks it into acetone, acetic acid, and 

 carbon dioxide. The power of hypochlorous acid in breaking 

 ring-structures renders it a reagent of great value in the hands of 

 the technical chemist. 



The story of the powers of modern chemistry might be in- 

 definitely continued, but may be summed up by saying that the 

 secret of the chemist's ability to synthesise such an infinitude of 

 substances, lies in the discovery of far-reaching chemical truths; 

 a discovery arising out of the study of structure and out of the 



