PROGRESS IN PHYSICAL CHEMISTRY. 235 



Laar put forward the view that the structure of a tauto- 

 meric compound is not definite, but is continually oscillating 

 from that of the one to that of the other tautomeric form. 

 Bruhl concludes that this view cannot hold for fatty com- 

 pounds, although it may be true for aromatic compounds. 



In another lengthy communication (35) Bruhl discusses 

 the constitution of benzene in the light of physical evidence. 

 He compares the densities at 20 , the molecular volumes, 

 and molecular refractions, of benzene and its di-, tetra-, and 

 hexahydrides, and of hexane, hexylene, diallyl, and dipro- 

 pargyl. The constants of the fatty compounds always differ 

 from those of their aromatic isomers, and the alteration in 

 the constants produced on passing from benzene to its hexa- 

 hydride is best explained by assuming that each addition 

 of H, corresponds with the loss of a double-linkage, and 

 that on passing from benzene hexahydride to hexane, the 

 large alteration in the physical constants is due to the 

 breaking up of the ring and the formation of a straight 

 chain. It is therefore concluded that Kekule's oscillation 

 formula best expresses the constitution of benzene, and it is 

 shown that thermal data may be interpreted as pointing to 

 the same conclusion. Bruhl considers that the structure of 

 benzene is best represented by Sachses model. This is a 

 regular octahedron with two parallel faces removed, six 

 regular tetrahedra having been placed on the six remaining 

 faces. The carbon atoms are supposed to be at the centres 

 of the tetrahedra, and the hydrogen atoms at the six apices 

 of the model. Kekule's formula is the best plane represen- 

 tation of this configuration. 



It is well to bear in mind, as Ostwald points out (36), 

 that the above treatment of the subject is in reality based 

 on analogy ; although it must be admitted that if the 

 attempt be made to apply the conceptions of double and 

 single bonds, Bruhl's conclusion is the most satisfactory. 



Mond and Nasini attributed the hioh molecular refraction 

 ot nickel carbonyl to the metal. Gladstone, however, con- 

 siders that the cause is to be found in the peculiar arrangement 

 of the carbonyl groups, and Armstrong has suggested that 

 when several of these are present in the molecule they exert 



