ABSORPTION SPECTRA AND CHEMICAL GONSTITUTION, 327 
On the Cause of Absorption Bands in the Spectra transmitted by Benzene and its 
Derivatives.) 
Tt was pointed out that in the absorption of ultra-violet rays by hydro- 
carbons of the aromatic series we have two kinds of absorption made 
manifest, namely, a general and a selective absorption, It is the selective 
absorption which distinguishes these from all other compounds of whatever 
class, so far as our knowledge at present extends. 
While the absorption spectrum of alcohols and fatty acids and amines 
depends upon and varies with the molecular weight of the compounds, or, 
more strictly expressed, with the number of carbon atoms in the molecule, 
in aromatic compounds it depends entirely on the structure of the molecule. 
For instance, a terpene, C,)H,,, is much more strongly diactinic than 
naphthalene, C,)H,, and benzene hexachloride, C,H,Cl,, than benzene, 
C,H,. Cymene, C,,H,,, or CH;'C;H,;CH(CH;)., p. methyl-isopropyl- 
benzene, if compared with naphthalene, C,,H,, is found to possess only 
one-fifth the absorptive power of the latter, and it gives a very different 
spectrum. There is a similar difference between anthracene and phenan- 
threne, both with the composition C,,H,). Such a difference is quite 
usual with isomerides of the aromatic series. 
When the molecule of a substance is capable of vibrating synchronously 
with a radiation, the ray received on this substance is absorbed. The ab- 
sorption is complete if the direction of the vibration of the ray and of the 
molecule is the same, but the phase opposite. It is evident that general 
actinic absorption, exerted by carbon compounds, is due to the vibrations 
of the molecules, since absorption increases in extent with the number of 
carbon atoms in the molecule, or, in other words, in any homologous series 
the greater the molecular mass the lower the rate of vibration of the 
molecule, Selective absorption appears to be caused by the vibration of 
atoms or atomic groupings within the molecule. 
When a substance such as benzene absorbs all rays more refrangible 
than \ 2743, it is because the molecules are vibrating synchronously with 
these rays, and the number of molecules within the path of the rays is 
sufficient to damp all vibrations. When the liquid is diluted the number 
of molecules present is not sufficient to damp all the vibrations, and some 
rays are transmitted. 
Tf, however, certain carbon atoms within the molecule are vibrating 
synchronously with certain rays, we shall have selective absorption of 
these rays after the general absorption has been so weakened by dilution 
as to allow them to pass. It was not found possible to associate any of 
the absorption bands of the substances examined with any particular 
carbon atoms ; furthermore, it was shown that the intra-molecular vibra- 
tions were dependent upon the vibrations of the molecules. 
From numbers representing approximately the mean wave-lengths of 
the four chief bands of rays absorbed by benzene, naphthalene, and 
anthracene, and from the velocity of light, the mean rate of vibration of 
the molecules of benzene, naphthalene, and anthracene were calculated, 
The following are the numbers given :— 
Mean A Vibrations per second of time 
Benzene, 2526 1248 10! 
ee 
Naphthalene, 2687 
Anthracene, 3439 S10" 1 
ow a 
1 Trans, Chem, Soe. vol, xxxix, p, 165, 1881, 
