346 REPORT—1899. 
zenes united in one molecule, will cause the invisible colours of the ultra- 
violet region to become visible. 
Triphenylmethane absorbs all the ultra-violet rays down to H, anthra- 
cene all the ultra-violet as well as a band of violet rays ; the former is 
therefore all but a substance with visible colour, the latter is undoubtedly 
a coloured substance. But bi-diphenylene-ethylene is strongly coloured, 
and the nature of its colour is such as to show that it absorbs the violet, 
blue, and green rays. These hydrocarbons constitute an interesting illus- 
tration of the passage from a substance which just falls short of being 
coloured to one which is but faintly, and to a third which is strongly 
coloured. 
H 
/ OH; ae C,H. C,H 
ean, eH, 4 ae. |, “00g ee 
\ CoH: pe OH, C,H, 
Researches on the Relation between the Molecular Structure of Carbon 
Compounds and their Absorption Spectra. Part VIII. A Study of 
Colowred Substances and Dyes. (HARTLEY.)! 
According to Dr. Otto Witt? the tinctorial character is conditional 
upon the simultaneous presence of a colour-producing group (chromogen) 
and asalt- forming group (chromophore) in the molecule. This investigation 
includes a study of the hydrocarbons in their relation to the more com- 
plex colouring compounds derived therefrom, considered in the light of 
Witt’s views. 
A perfectly colourless substance transmits all luminous and invisible 
vibrations without impairing their intensity ; a coloured substance absorbs 
rays at either end of the spectrum, even beyond the limits of visibility, or 
say from \ 7800 to \ 2000, or it selects rays from the middle of the spec- 
trum. Every fluorescent substance is therefore in a certain sense 
coloured, because it absorbs certain rays whether in the visible or 
ultra-violet region. Benzene, benzenoid hydrocarbons, phenols, &c., which 
exhibit selective absorption, are also coloured, although the eye, owing to 
absorption of the ultra-violet rays by the aqueous humour,’ has a range of 
vision limited by the red and violet ends of the spectrum, and therefore can- 
not appreciate this variety of colour. Bands of selective absorption, it has 
been shown, are to be attributed to the effect of vibrations taking place within 
the molecules of a substance upon the rays which enter the substance, and 
are dependent upon the rate of vibration of the molecules themselves. To 
convert a carbon compound, therefore, such as benzene—which, owing to its 
powerfulabsorption in the ultra-violet, may be said to have invisible colowr— 
into a compound with visible colowr, it is only necessary to slacken its rate 
of vibration so that the molecule will absorb rays with oscillation fre- 
quencies occurring within the limits of visibility. Or, to put it in another 
way, the absorption band in the ultra-violet is transferred to rays of 
iower refrangibility. A chromogen is an invisibly coloured substance ; a 
chromophore is an atom, or group of atoms, capable of reducing the rate 
1 Trans. Chem. Soc. vol. li. 1887, p. 153. 
2 Ber. vol. ix. p. 522. 
3 J. L. Soret, Comptes Rendus, vol. xcvii. p. 572. 
——— ae 
