ON ABSORPTION SPECTRA AND CHEMICAL CONSTITUTION. 



225 



The remark of Bayrac and Camichel that indophenols with tertiary 

 nitrogen groups are much more absorbent than those with primary 

 nitrogen is only what might be predicted from what we know of the 

 ultra-violet spectra. The homologues of benzene, such as toluene, ethyl- 

 benzene, and the xylenes, are more powerfully absorbent than benzene 

 itself. The tertiary monamines trimethylamine and triethylamine are 

 more absorbent than the corresponding primary bases. Moreover, it was 

 proved in the case of dyes that in the triphenylmethane derivatives the 

 replacement of 3H by (0113)3 rendered the substance much more power- 

 fully absorbent, methyl violet and rosaniline hydrochloride being a case 

 in point. This is best shown by the curves which illustrate the original 

 paper ; but it also appears from the measurements which have already been 

 quoted, if we consider that the red rays are freely transmitted by the 

 rosanile salt when even stronger solutions than those containing a milli- 

 gramme-molecule of substance in 100 c.c. The methyl derivative 

 barely transmits any light through 25 mm. of such a solution. Then, 

 again, the width of the band transmitted by the methyl violet is narrower. 

 The same observation applies to iodine green. 



The mere position of a band of transmitted red rays cannot be 

 considered as indicative of a constitution similar to that of the triphenyl 

 methane derivatives or of the indophenols because many of the diazo- 

 colours show such a band. The difference between them lies in the effect 

 of dilution ; in fact it is the absorption curves which are of importance, 

 or, better still, the curves of molecular vibrations. There is a particular 

 curve for each class of derivatives, the particular members of each class 

 showing variations of the curve characteristic of the class. This is more 

 marked in the case of the azobenzene and azonaphthalene derivatives 

 than it is even in the derivatives of triphenylmethane, because a larger 

 number of individual substances belonging to the former class have been 

 examined than of the latter. It is quite evident that the nitrogen groups 

 are chiefly concerned in the development of the colours, and the hydro- 

 carbon radicals appear to be of comparatively small importance provided 

 they are of a benzenoid character. 



APPENDIX. 



List of Substances the Absorption Spectra of tvhich have been studied in 



connection ivith the Chemical Constitution of Organic Compounds. 



Note. — The method of indexing adopted by the Chemical Society has been followed. 



