CHEMISTRY OF THE NINETEENTH CENTURY. 633 



In the first arrangement no carbon atom is joined to more than 

 two other carbon atoms ; in the second, one of the carbon atoms is 

 directly connected with three others. If we take the next higher 

 member of the (a) group, C 5 H I2 , we shall find on trial that three dif- 

 ferent arrangements are possible ; viz., (a) one in which no carbon 

 atom unites with more than two others ; (/3) one in which one of the 

 carbon atoms unites with three others ; and (7), one in which one of 

 the carbon atoms unites with four others. We here represent these 

 three modifications as before, only substituting for the black and white 

 circles the literal symbols of carbon and hydrogen respectively, which 

 is the more usual mode of exhibiting these "graphic formulae," as they 

 are called. 



H H H H H 



H-C-C-C-C- 

 H H H H H 



-H 



C S H I3 (a) 



H 



H 



H 



H 



H C H 



H C-H 



H 



H 



H C H 

 H H 



H C C C H 



H H 



H C H 



C 5 H I 



Or, writing the symbols in a more condensed form, while retaining 

 the same notion of their construction, we may exhibit them thus : 



(a) CH 3 . CH 2 . CH 2 . CH 2 . CH 3 ; 



CH 3 . CH 2 . CH(CH 3 ) 2 ; 



(y) c(CH 3 ) 4 . 



Now, chemists are acquainted with a vast number of instances in 

 which substances of more or less different properties have precisely 

 the same per-centage composition, and have also in the state of vapour 

 the same specific gravity; that is, their molecules are formed of the 

 same number of atoms of the same kind. The difference of their 

 properties can, therefore, only be accounted for by a different arrange- 



