68 
Proceedings of the Royal Society of Edinburgh. [Sess. 
IV. — On a Question in Absorption Spectroscopy. By Robert A. 
Houstoun, M.A., Ph.D., D.Sc., assisted by Alexander S. Russell, 
M.A. Communicated by Professor A. Gray, F.R.S. 
(MS. received October 12, 1908. Read November 2, 1908.) 
[n the third volume of his Spectroscopic, p. 91, Kayser has raised the 
question whether on mixing two coloured solutions which do not act on one 
another chemically the absorption spectrum of each of the components 
remains unchanged. Melde thought he had discovered such an effect ; he 
stated that when a solution of carmine in ammonia which has two sharp 
bands in the green, was added to a solution of potassium dichromate which 
absorbs the violet end of the spectrum, or to an ammoniacal solution of 
copper sulphate which absorbs the red end, that the carmine bands were in 
each case displaced towards the end absorption in question. He ascribed 
this to a physical action between the molecules. It was, however, pointed 
out by Schuster that a shift of this nature would be seen if, instead of 
mixing, the one solution was merely placed behind the other. Bostwick * 
and Kriiss f repeated Melde ’s work, and came to the conclusion that there 
was a real shift in addition to the apparent shift pointed out by Schuster. 
Since then additional evidence has been adduced by FormanekJ and has 
been quoted by Kayser in the section cited above. The object of the 
research recorded in this paper was to investigate those cases with the 
most accurate means possible, and, if a shift was established, to decide if it 
was physical. 
We have theoretical grounds for expecting such a shift. Suppose that 
we have an electric doublet vibrating according to the equation 
<*> 
where x specifies the position of the negative electron relatively to the 
* A. E. Bostwick, “ Preliminary Note on the Absorption Spectra of Mixed Liquids,” Amer . 
J. (3), xxxvii., pp. 471-473 (1889). 
t G. Kriiss, “ Ueber die Constitution von Losungen,” Ber. Chem. Ges., xv., pp. 1243-1249 
(1882). 
f J. Formanek, “ Untersuchung und Nachweis Organischer Farbstoffe auf Spectroscopi- 
scbem Wege,” Zs. f. Farben- u. Textil-Chemie, i. Heft 11 (1902). 
