CLASSIFICATION OF ORGANIC SUBSTANCES. 



303 



THE OLEFINES, OR ETHYLEXE SERIES OF HYDROCARBONS. 



420. Homologues and Isologues. The members of a group 

 of hydrocarbons which differ regularly by CH 2 , as in this 

 table, are said to be homologues, or to form a homologous 

 series. Two or more series, on the other hand, differing 

 from each other by H 2 , are said to be isologites, or to form 

 an isologous series. Thus the members of the Ethylene or 

 Olefiant Gas Series are homologous among themselves, but 

 isologous with respect to the Marsh Gas Series on page 

 324. Many of the hydrocarbons of such isologous series 

 are rare bodies, mere chemical curiosities; but we will 

 give you a table showing some of these, that you may the 

 better understand the terms homologous and isologous, and 

 that you may see at a glance the enormous number of com- 

 pounds of hydrogen and carbon which are capable of ex- 



