ABSORPTION SPECTRA AND CHEMICAL CONSTITUTION. 317 
corresponding to each increment of CH, in the molecule of the alcohols 
and acids. 
4. Like the alcohols and acids the ethereal salts (esters) derived from 
them are highly transparent to the ultra-violet rays, and do not exhibit 
absorption bands. 
At a later date it was shown incidentally in the examination of other 
substances that the various secondary alcohols, isopropylic, isobutylic, and 
the different amylic alcohols showed no absorption bands. The same 
properties were shared by various polyhydric alcohols, such as glycol, 
glycerine, mannite, cane sugar, and dextrose. 
In fact no open chain carbon compound causes selective absorption. 
All substances derived from a closed chain of carbon atoms of the 
benzene type were found to be strongly adiactinic. It had been shown 
by Sir George Stokes that one of these substances, salicine, developed an 
absorption band when the solution was diluted. This matter was followed 
up by the examination of allied substances such as phenol, salicylic acid, 
and other derivatives of benzene, to ascertain whether they also exhibit 
absorption spectra characterised by bands. 
The facts elicited were the following :— 
5. Benzene and the hydrocarbons derived therefrom by the replace: 
ment of hydrogen, phenols, aromatic acids and amines, are remarkable— 
first, for their powerful absorption of the most refrangible rays ; secondly, 
for the absorption bands made visible by dissolving them in water or 
alcohol ; and thirdly, for the extraordinary intensity of these absorption 
bands even in very dilute solutions. 
6. Isomeric substances containing the benzene nucleus exhibit widely 
different spectra, inasmuch as their absorption bands vary in position and 
intensity. 
7. The photographic absorption spectra can be employed as a means of 
identifying organic substances, and as a most delicate test of their purity. 
The curves obtained by co-ordinating the extent of dilution, or in other 
words the quantity of substance, with the position of the rays of the 
spectrum transmitted by the solution, form a strongly marked and highly 
characteristic feature of very many substances. 
Observations were extended to the essential oils, because they are 
known to consist for the most part of hydrocarbons which are physically 
isomeric, and which differ therefore in constitution from benzenoid hydro- 
carbons and their derivatives, though in a manner closely related to 
benzene. A large number of specimens were thoroughly examined.! 
According to the classification employed at that time there were three 
groups, the second and third being polymers of the first, as shown by their 
formule, 
CioHi¢, Cy5H2y, and CyoH3p. 
No absorption bands were discovered in any of the following specified 
substances or in the hydrocarbons derived from them by distillation ; but 
the extent of the absorption of the ultra-violet rays was found to be 
greater the larger the number of carbon atoms in the molecules ; such 
increase in the number of carbon atoms being due to the side chains 
when the nucleus of the compound was of the ring-form :—Terebene 
' Proc. Roy. Soc. yol. xxxi. pp. 1-26, 1881, Hartley and Huntington. 
