1910-11.] Absorption of Light by Inorganic Salts. No. I. 521 
XXXVI. — The Absorption of Light by Inorganic Salts. No. I. : 
Aqueous Solutions of Cobalt Salts in the Infra-Red. By 
R. A. Houstoun, M.A., Ph.D., D.Sc., Lecturer on Physical Optics in 
the University of Glasgow. Communicated by Prof. A. Gray, F.R.S. 
(MS. received March 8, 1911. Read June 5, 1911.) 
The present article is intended to be the first of a series on the absorption 
of light by solutions of inorganic salts of different elements. With a few 
scattered exceptions all the work hitherto done on the absorption spectra of 
inorganic salts has been merely qualitative and has been confined to the 
visible spectrum. Kayser in his Spectroscopie, vol. iii. p. 45, states that in this 
field there is work for years and for numerous observers. E. C. C. Baly states 
in his Spectroscopy, p. 407, that not much is known about the absorption of 
light in inorganic salts. Merely for its own sake, then, an accurate deter- 
mination of the molecular extinction coefficient for as many salts under as 
many different conditions of temperature and concentration, and for as many 
wave-lengths as possible, would be very valuable. 
But the molecular extinction coefficient is also of great theoretical 
interest. Clerk Maxwell said that the constitution of the atom was acces- 
sible to the student who was armed with the spectroscope and the calculus. 
The emission spectrum, could we only read it, was a key to the structure of 
the atom. But although we are now able to measure a wave-length to an 
infinitesimal fraction of a centimetre, comparatively little progress has been 
made since Maxwell s time in reading emission spectra. Some of the spectra, 
too, have so many thousand lines that the prospects of constructing a 
dynamical system with degrees of freedom to correspond to the lines appears 
hopelessly remote. Emission spectra vary considerably with the means of 
production, and with the coming of radioactivity there has come also the 
idea that the emission spectrum is not so much a key to the structure of the 
atom as a key to the manner in which the atom is being destroyed. In order 
to get at the structure of the atom we must study its spectrum under normal 
circumstances at ordinary temperatures and under specified conditions that 
we can easily reproduce. That is, we must take the absorption spectrum of a 
solution. The molecular extinction coefficient may thus help to a knowledge 
of the structure of the atom. 
The molecular extinction coefficient is defined by the following equation, 
which also gives the law according to which light is absorbed by a solution : 
I = I 0 10- Acci . 
