ABSORPTION SPECTRA AND CHEMICAL CONSTITUTION. 345 
(2) The substitution of G-naphthol disulphonic acid 8, or 6-naphthol 
disulphonic acid R for #-naphthol sulphonie acid B, causes a shifting of 
the bands, which in the case of B-naphthol sulphonic acid S amounts to 
from 4 to 5 millionths of a millimetre. 
(3) In the substitution of methyl the space between the two bands 
becomes clearer, and the bands become more equal in intensity and width. 
The substitution of B-naphthol sulphonic acid S or B-naphthol di- 
sulphonic acid R, in the place of the acid B, acts in a similar manner on 
the character of the bands. The solutions were made both with alcohol 
and with strong sulphuric acid. 
OBSERVATIONS ON THE ORIGIN OF COLOUR AND ON FLUORESCENCE.! 
Anthracene is shown to possess a very definite absorption band in the 
violet, and it is therefore a truly coloured substance. Its colour is the 
aggregate effect on the retina of those rays which are the complement in 
white light of the violet rays absorbed. The complementary colour to 
the violet can be seen only by transmission. Hence the colour which is 
due to fluorescence must be excluded from access to the eye, and also 
all reflected light. It is therefore necessary to look directly through thin 
layers of the substance, or to view it by transmitted light when the sub- 
stance is fused. When examined in this manner, anthracene is seen even 
in very thin layers to have a greenish yellow colour. This colour belongs 
to only the very purest specimens. 
Nietzki has stated that all hydrocarbons are colourless,” but this is 
not strictly the case. Anthracene is not the only hydrocarbon which 
possesses colour visible to the human eye. Dela Harpe and Van Dorp * 
obtained a red hydrocarbon, C,,H,,, by passing fluorene over heated lead 
oxide. Mantz* established its molecular weight by Raoult’s method, and 
assigned it the structural formula 
C,H 
: 6 Negi 
ot, \é yt, 
C. Grebe’s investigation ° shows that the colour truly belongs to the 
substance, and that it becomes brighter in colour the more highly it is 
purified. He attributes the colour to the grouping >C:C<,as both the 
dibromide and the hydrocarbon C,,H,, are colourless. 
It may be remarked that the carbon grouping > C:C< cannot alone 
be the cause of the colour, for if the carbons are united to H atoms or to 
alkyl radicals it may be safely predicted that no colour would result. It 
is this nucleus united to four benzene rings, each pair of which are them- 
selves united, and so forming molecules with a great compactness of 
structure, which is the cause of the colour. Benzene is a substance with 
invisible colour, that is to say it exerts a powerful selective absorption in 
the invisible region, and by any series of chemical reactions which serve 
to retard the rate of vibration of the molecule, or of any group of ben- 
! Trans. Chem. Soc. 1893, p. 243, Hartley. 
2 Page 2 of the English edition of his work. 
8 Ber. vol. viii. p. 1048. 
* Inaugural Dissertation, Geneva, 1892. 
° Ber. vol, xxv. pp. 3146-49. 
