NEW LAW IN THERMOCHEMISTRY 



23 



the molecule less one. In the ring structure, if we ignore 

 double bonds, n is equal to or greater than the number of 

 atoms. Thus in benzene, CgHg, n is at least 12, whereas in its 

 isomer, dipropargyl, « = 11. If now we provisionally assume 

 that in the aromatic compounds the number of linkings and the 

 number of atoms are equal, we shall find that the regular for- 

 mula applies to them also, provided that a constant correction 

 be applied. This correction in the quadrupled equation, is 

 — 12, and the general formula for the aromatic hydrocarbons 

 becomes 



4^ 



12a + 6b — c — Sn — 12 



= constant. 



For other compounds, like phenol, chlorobenzene, aniline,"etc., 

 other factors must be included, exactly as in the previous series. 

 These factors, //, gvi, — Cp 95, etc., have already been ade- 

 quately explained. The data, all due to Thomsen, are as fol- 

 lows, with the results of computation added : 



Compound. 



Benzene 



Toluene 



Mesitjlene 



Pseudocumene 

 Chlorobenzene 



Phenol 



Anisol 



Aniline 



Pyridine 



Thiophene 



Formula. 



CjHg 



C H 



CjHjj 



CeHjCl 



CeHeO 



C^HgO 



CeH.N 



C5H5N 



^K. 



HjO Liquid. 



3151800 

 3822720 

 5129240 

 5 1 26040 

 3055520 

 3075040 

 2745200 

 3353880 

 2700282 

 2442560 



H,0 Gas. 



3026724 

 3655982 

 4879082 

 4875888 

 215 1 290 

 2949964 

 3578432 

 3207958 

 2596050 

 2359176 



Divisor. 



Quotient. 



222 



264 

 348 

 348 

 213 

 214 

 258 

 235 

 193 

 172 



Mean, 13782 



13634 

 13812 

 14020 

 14011 

 13856 

 13785 

 13884 

 1365 1 

 13451 

 13716 



For benzene I have taken Thomsen's older determination,^ 

 which gives the heat of combustion as 787950 calories. His 

 later determination^ of 799280 has been shown by Stohmann 

 and by Berthelot to be too high. Their measurements agree 

 with the lower value. The higher figure probably owes its 

 excess to extraneous heat derived from the use of Thomsen's 

 *' universal burner." 



1 Therm. Unt., ii, 95. 

 ^Ibid.,i\, 59. 



