ABSORPTION SPECTRA AND CHEMICAL CONSTITUTION. 351 
have already been described. The striking examples of isomerism re- 
marked in the tropxolines, with corresponding differences in the curves of 
their molecular vibrations, suggested a systematic examination of some of 
the simpler isomeric derivatives of benzene as being an interesting subject 
for investigation. 
Researches on the Relation between the Molecular Structure of Carbon 
Compounds and their Absorption Spectra. Part IX. (Hartyey).! 
On Isomeric Cresols, Dihydroxybenzenes and Hydroxybenzoic Acids. 
The results of the examination of the absorption spectra of (i.) ortho-, 
meta- and para-cresol ; (ii.) hydroquinone (quinol), pyrocatechol, and 
resorcinol ; (iii.) salicylic acid, metahydroxybenzoic acid, and parahydroxy- 
benzoic acid, are given. 
The oscillation frequencies of the most extreme rays transmitted by a 
milligram-molecule of the four classes of isomeric substances, are the 
following :— 
- ; : Hydroxybenzoic / 
x Cresol; Dihydroxyb ‘ 
ylenes resols ihydroxybenzenes Acids 
Ortho. . 3611 | Meta. . 3433 | Meta. . 3466 | Para . . 3359 | 
Meta . . 3580 | Ortho. . 3413 | Ortho. . 3399 | Meta . . 3080 | 
Para . Monon.) bara ~ ; aang | bara. 3) olpile}Ortno: o- . 2986 | 
Tt is seen, therefore, that in the case of the xylenes, cresols, and 
dihydroxybenzenes, the 1 : 4 derivatives exercise the greatest absorption. 
It is also to be noted that the cresols and dihydroxybenzenes follow the 
same order of transparency, while in the case of the hydroxybenzoic 
acids this order is reversed, and in the case of the xylenes it is different 
from that of any of the other three series. 
On the assumption that the absorption of rays manifested by any 
three isomerides is a measure of its rate of vibration, and consequently of 
the dissipation of energy resulting in the formation of the molecule, the 
following classification may be deduced :— 
Dissipation of Energy during Formation. 
Least Greatest 
Xylenes Para Meta Ortho 
Cresols Para Ortho Meta 
Dihydroxybenzenes Para Ortho Meta 
Hydroxybenzoic Acids Ortho Meta Para 
It is worthy of note that v. Rechenberg,” from the heat of combustion 
of the three hydroxybenzoic acids, places them in the same order as that 
given above. 
Description of the Spectra of the substances examined. 
Ortho-cresol, C,H, (CH,)*OH. 1: 2. 
‘108 gram in 20 ¢.c. alcohol. 
With 5, 4, 3, 2 and 1 mm. an absorption band appears from !1/\ 3413 
to 4125, which gets narrower as the thickness of the layer of liquid is 
reduced. 
1 Trans. Chem. Soc. vol. liii. p. 641, 1888. 
2 J. vr, Chem. vol. =xii. pp. 1-45. 
