-7^ Appendix. 



The substitution of methyl, CH3, for one, two, three, etc., 

 of the atoms of H (around the hexagon) — and also for atoms of 

 H in CHg — constructs the /iydrocaH)ons oi this group : 



C„H2o_6 Number of possible Isomers. 



1. CsHo Benzene. 



2. C6H3(CH3)=C,H3 Toluene. 



3. CeH4(CH02 = C8H,o Xylene. Three(i, 2 : 1,3:1,4). 



4. Ci!H3(CH.)., = C9Hjo Cumene. Three(i, 2, 3:1, 2, 



[4: I, 3' 5)- 



5. C6H2(CH.,)^ = CioHiiCymene. Threed, 2, 3, 4:1, 



[2, 4, 5 : I5 3' 4^ 5)- 



6. C„H(CH3)5 = Ci,His Amylbenzene. One(i, 2, 3, 4, 5). 



7. Cp (CH3 ),; = Ci2Hi,, Amylmethylbenzene. 

 etc. 



4 The hydrocarbon having two of the or- 



iginal hydrogen atoms displaced (no. 3) 



may evidently have contiguous or alternate 

 3 



or opposite atoms displaced, thus present- 

 ing three different kinds of molecules, — and 



the number of variations mathematically 

 2 



possible is given above as a theoretical num- 

 ber of possible isomers. The full number 

 I of possible isomers, on this theory, has 



been actually produced in most instances 

 though not in all, and perhaps has not been exceeded in any in- 

 stance. Thus, there are known three compounds having the 

 ultimate composition and molecular weight of xylene, differing 

 in certain properties : orthoxylene (having parts i and 2 of the 

 hexagon occupied by methyl), isoxylene (having i and 3) and 

 metaxylene (having i and 4 occupied by methyl). This corre- 

 spondence between fact and theory in the number of isomers 

 strengthens the evidence that the hexagonal figure represents the 

 actual relations of the atoms in the molecule. But it must be 

 borne in mind that, while figures are placed upon paper to rep- 

 resent certain ascertained relations of atoms to each other, noth- 

 ing has been ascertained as to the places of atoms in molecules. 



