288 CONSIDERATION OF CARBON COMPOUNDS. 



chains. By saturating, for instance, the affinities in the open carbon 

 chains mentioned above, we obtain the compounds CH 4 , C 2 H 6 , C 3 H 8 , 

 C 4 H 10 , etc. 



H HH HHH HHHH 



H C H, H C-C H, H C C C H, H C C C C H, etc. 



H HH HHH HHHH 



Many homologous series of various organic compounds are known, 

 as, for instance : 



C H 3 Cl, C H 4 O, C H 2 O 2 . 



C 2 H 5 C1, C 2 H 6 0, C 2 H 4 2 . 



C 3 H 7 C1, C 3 H 8 0, C 3 H 6 2 . 



C 4 H 9 C1, C 4 H 10 0, C 4 H 8 2 . 



C 5 H n Cl, C 5 H 12 0, C 5 H 10 2 . 



etc. etc. etc. 



Types. It has been proposed to select some substances in which the arrange- 

 ment of atoms in the molecules may betaken as representative of whole classes 

 of other substances, the molecules of which have a similar arrangement, and 

 these normal substances have been termed types. Most substances may be 

 classified under the following five types : 



By replacing the constituents of these types by other elements or radicals of 

 equal valence, most of the compounds known (both organic and inorganic) may 

 be classed in one of these types. 



The following five substances may, for instance, be said to have an atomic 

 arrangement similar to the types above stated : 



I. II. III. IV. V. 



Hydrochloric Potassium Arsenous Ethane. Phosphoric 



acid. hydroxide. chloride. oxychloride. 



CH 3 O 



/Cl 



H Cl, K O H, As^-Cl, 



^Cl 



H Cl 



3 



| H || Cl 



Of P( 



| X H | X C1 



The graphic representation of the constitution of compounds according to 

 types has greatly aided in disclosing their structure, and is frequently used to 

 give a picture, as it were, of the theoretical views held regarding the atomic 

 arrangement. 



Substitution is a term used for those reactions or chemical changes 

 which depend on the replacement of an atom or a group of atoms by 



