and on the Constitution of Benzene. 



429 



From this it will appear that mesotartaric acid for example 

 is to be regarded as a geometrical isomer of dextro- and 

 laevotartaric acids, which two are optical isomers of one 

 another. Maleic and fumaric acids, again, are geometrical 

 isomers of which optical isomers are wanting. This iso- 

 merism, whether optical or geometrical, is due to the 

 presence of carbon atoms, whose images are different from the 

 original atoms, carbon atoms in short which are asymmetric. 



I come now to the consideration of the constitution of 

 benzene and to the expression of its constitution on the 

 assumption of the tetrahedric carbon atom. 



Former attempts in this direction have led rather to a* 

 preference being assigned to the symbol of Prof. Kekule, 

 of alternate double and single linking, and this symbol can 

 readily be represented on the tetrahedric formation (fig. 6). 



Fig. 6. 



Note.— In the figures of the models (6, 8, 9, 11) the carbon atoms are 

 represented by the tetrahedrons, the hydrogen atoms are not given but 

 are supposed to occupy the free angles of the tetrahedrons. 



Such a formula perhaps brings into prominence a grave 

 objection to Kekule's symbol as to the positions 1 : 2 and 

 1 : 6 being identical, and shows the difficulty of assuming 

 the double and single linkings to change places with one 

 another. 



But the symbol of Kekule, and with it its geometric inter- 

 pretation, may be said now almost to have received their 

 death-blow at the hands of Prof. v. Baeyer. In his work 

 on the hydroterephthalic acids {Ann. ccxlv. p. 103) he shows 

 that these acids act as fatty compounds, the hexa acid as a 

 saturated, the tetra and dihydro acids as unsaturated fatty 

 compounds, and that, indeed, in the latter two acids it is 

 possible to determine between what carbon atoms the double 

 linkings lie. But when we come to terephthalic acid itself 



Phil. Mag. S. 5. Vol. 20. No. 162. Nor. 1888. 2 G 



