316 - REPORT—1899. 
Absorption Spectra and Chemical Constitution of Organic Substances. 
—Inierim Report of the Committee, consisting of Professor W. NOEL 
HartLey (Chairman and Secretary), Professor F. R. Japp, and 
Professor J. J. DOBBIE, appointed to investiqate the Relation 
between the Absorption Spectra and Chemical Constitution of Organic 
Substances. 
Introduction. 
In presenting an interim report on the subject of the relation between the 
Absorption Spectra and Chemical Constitution of Organic Substances, it 
will be convenient to refer briefly to the report made to the British 
Association which was drawn up by Professor Huntington and presented 
at the Swansea Meeting in 1880.! It will there be noticed that the work 
originated in the discoveries of Sir George Gabriel Stokes in 1852 and 
1853, and of the late Dr. William Allen Miller in 1862. Next M. L. 
Soret and MM. Soret and Rilliet advanced this line of research by 
showing that by the increased molecular mass of the alkyl radical there 
was increased absorption of the ultra-violet rays, though no absorption 
bands were observed in nitrates and nitrites of these substances. W. N. 
Hartley, in 1874, from a consideration that all the characteristic physical — 
properties of organic substances are dependent on their molecular consti- 
tution, inferred that if a large number of substances of a similar con- 
stitution were examined, such as the ethereal saits of the organic acids 
and homologous series of the normal alcohols and acids, evidence would be 
obtained of the influence of impurities and of the variations in the absorp- 
tion of the invisible rays caused by each increment of CH, in the molecule. 
The work was found impracticable without the aid of photography, and 
a form of camera was therefore constructed which admitted of metallic 
spectra being taken with all lines in focus on a flat plate from wave- 
lengths 5,400 to 2,000. It was also found necessary to employ dry-plates, 
and all the known makes were tried, some of which proved to be quite 
unsuitable, For the first time in spectrum work gelatino-bromide plates 
were used with success. The method of experimenting at the present 
time, except for a few modifications, is that described in the ‘ Phil. Trans.,’ 
Part I., 1879., Hartley and Huntington.? 
After the examination of a large number of specially purified carbon 
compounds the following generalisations were arrived at :— 
1. The normal alcohols of the series C,,H,,,,,OH are remarkable for 
transparency to the ultra-violet rays, pure methylic alcohol being nearly 
as much so as water. 
2. The normal fatty acids exhibit a greater absorption of the more 
refrangible rays than the normal alcohols containing the same number of 
carbon atoms. 
3. There is an increased absorption of the more refrangible rays 
1 Report of the Fiftieth Meeting. (See p. 303.) 
2 Proc. Roy. Soc., 1879, pts. i.and ii., vol. xxviii., p. 233. Scientifie Proceedings of 
the Roy. Dublin Soc., vol. iii., p. 93 (new series). ‘ Description-of the Instruments 
and Processes employed in Photographing Ultra-Viglet Spectra,’ 1881, Hartley, 
