450 Mr. S. A. Sworn on the Constitution 



(b) This explanation is dependent upon the nature of the 

 radical first introduced. If this radical has no tendency to 

 form addition products, the further substitution must be 

 effected by the momentary dissolution and re-establishment 

 of one of those aromatic linkages which attach its ortho- and 

 para- neighbours to the carbon atom whose hydrogen is al- 

 ready displaced. (This explanation is admitted for the mono- 

 substitution derivatives.) 



Meta-derivatives are not obtained because there is no meta- 

 linkage, and para-derivatives are formed in greater quantity 

 than ortho- because the para- is less stable than the ortho- 

 linkage. The formation of o- and £>-dibromobenzenes, by the 

 direct bromination of benzene, may be thus represented. (See 

 page 451.) 



(c) In a similar development of Meyer's symbol ih.e ortho- 

 hydrogen atoms would be nearest to one another, and similar 

 arguments would lead us to ortho- and para-laws, and the 

 formation in each case of meta-compounds in small quantity. 



Attempts have been made by Schiff (Annalen, ccxx. p. 303), 

 by Lossen (Annalen, ccxxv. p. 119), by Horstmann (Ber. 

 xx. p. 766; xxi. p. 2211), by Briihl {Annalen, cc. p. 228), 

 by Thomsen [Therm. Unt. iv. pp. 61, 273), and others 

 to determine the constitution of benzene from its physical 

 constants. The calculations of these chemists presuppose the 

 existence of parafhnoid and olefmoid linkages alone, and lead 

 to conflicting results. Moreover they are based on so-called 

 " laws," such as that of Kopp, the general application of which 

 (in the case of molecular volumes) has been disproved by 

 numerous researches. 



It is certainly true that the linkages in aromatic com- 

 pounds are not directly comparable in such a manner with 

 those in fatty bodies ; and it is therefore impossible to make 

 use of arguments based upon the measurements of specific 

 volumes and refractive indices and upon thermochemical 

 data, until some quantitative connexion has been made 

 out. 



In a future paper I hope to bring forward some calcula- 

 tions based upon space formulas and made with the object of 

 advancing this question. 



