Influence of Atomic Weight. 377 



simple multiple of the chemical or true molecular weight ; n 

 is a number proportional to the number of liquid molecules in 

 a given unit of length. 



Melting-points of Carbon Compounds in general. — But little 

 attention has been directed to the influence of atomic weight 

 on and the relations between the melting-points of carbon 

 compounds. It is, however, a subject which would no doubt 

 repay careful investigation, and is especially important from 

 the fact that this physical property is the one which renders the 

 most service in the recognition of the solid compounds of 

 carbon. The following relations will be of interest. . 



As pointed out above in speaking of boiling-points, those 

 aromatic hydrocarbons containing an even number of methyl 

 groups are solid, whilst those containing an odd number are 

 liquid. A similar thing applies to the chlorine, and doubtless 

 to other derivatives of benzene ; for those containing an odd 

 number of 01 atoms melt much lower than those with an 

 even number, thus : — 



1 C 6 H 6 . 



C e H 5 .Cl. 



C 6 H 4 . C1 2 J 6 H 3 . Cl 3 . 



C 6 H 2 .ClJc 6 H.Cl 5 . 



c 6 ci 6 . 



Melting- \ _ 

 point, J ~~ 



3° 



-40° 



53° 17° 



139° 74° 



1 



26° 



The difference between those compounds containing an odd 

 number of CI atoms is constant, viz. 57°, and also the differ- 

 ence between those containing an even number of CI atoms, 

 viz. 86°. These facts, as previously intimated, probably de- 

 pend on the symmetrical or asymmetrical arrangement of the 

 CI atoms. 



As a general rule, the melting-points of a series of homo- 

 logous compounds rise as we ascend in the series ; but there 

 is one remarkable case known in which the reverse is the 

 case, thus : — Melting-point. 



Methyl terephthalate 140 



Ethvl „ 44 



Propyl „ 31 



Butyl „ liquid 



Isopropyl „ 56 



Isobutyl „ 53 



The relations between the melting-points of organic com- 

 pounds and their chemical composition offer so wide a field of 

 investigation, and one which has been so little touched, that 

 it is my intention at an early date to trace these relations as 

 completely as possible, especially as regards the influence of 

 the position of the atoms, and more particularly with respect 

 to the influence of symmetry. I have already worked a good 



