on the Physical Properties of Compounds. 185 



These relations as regards the heats of formation and 

 combustion of isomeric compounds are thrown out merely as 

 suggestions; for the data are so extremely few in number that 

 I should not be justified in doing more. 



I may here mention that Julius Thomsen has shown that 

 the determination of the heat of combustion will in many 

 cases enable us to say whether two or more carbon atoms are 

 united by single, double, or more linkings. Thus, he has 

 shown that ethane is CH 3 — CH 3 , and propane CH 3 . CH 2 . CH 3 , 

 i. e. that their carbon atoms are united by single links only ; 

 whilst in ethylene and propylene two carbon atoms are con- 

 nected by double linkings, thus, CH 2 =CH 2 , and CH 2 = 

 CH . CH 3 ; whereas in acetylene the two carbon atoms are 

 bound by treble linkings, CH=CH. Thomsen also concluded, 

 from the heat of combustion, that the six carbon atoms in ben- 

 zene are not bound by three double and three single linkings, 

 thus, 



C 



-o' V 



Y 



but by nine single linkings, thus: — 



CI 



-HC CH- 

 -HC CH- 



CH 



Bruhl, however, as we shall see presently, concludes from the 

 specific refraction of benzene that the old yiew is the correct 

 one. 



IY. LsTLUEXCE OF AT03IIC AbRAKGEMEMT OX OPTICAL 

 PiOTATTOX. 



According to Pasteur {Researches, p. 27), molecules are 

 diyisible into two classes: — (1) those in which the atoms are 

 symmetrically arranged, (2) those in which the arrangement 

 is asymmetrical. The property of optical activity belongs to 

 the latter class only; and Le Bel has proposed the hypothesis 

 that when a carbon atom is united directly with four different 



Phil Mag. S. 5. Vol. 13. No. 80. March 1882. Q 



