L 601 J 
XXIX. Photo-chemical Pesearches. — Part III. Optical and Chemical Extinction of the 
Chemical Rays. By Egbert Bunsen, Professor of Chemistry at the University of 
. Heidelberg, and Henry Enfield Koscoe, B.A., Ph.I)., Professor of Chemistry at 
Owens College, Manchester. Communicated by Professor Stokes, Sec. R.S. 
Received May 20, — Read June 18, 1857. 
In order to determine whether the act of photo-chemical combination necessitates the 
production of a certain amount of mechanical eifect, for which an equivalent quantity of 
light is expended, or whether this phenomenon is dependent upon a restoration of 
equilibrium effected without any corresponding equivalent loss of light, we must study 
more specially the phenomena occurring at the bounding surfaces, and in the interior of 
a medium exposed to the chemically active rays. 
A certain large class of bodies permit the chemical rays to pass through them to a 
greater or less extent, whilst other substances are opake to these rays. The first class, to 
which the name “ diactinous ” may be given, includes almost all colourless, blue, and 
violet media ; the second class of “ anactinous ” bodies consists chiefly of opake, and 
yellow or red substances. Although the terms “ diactinous ” and “ anactinous,” like 
“ diathermanous,” “ diaphanous,” &c., merely represent phases of the same difference, 
and can therefore bear no strict scientific definition, yet we do not hesitate to employ 
these terms as a means of avoiding a tedious circumlocution. 
When a ray of light penetrates a body which is not photo-chemically sensitive, a part of 
the hght is absorbed, and an equivalent quantity of heat evolved. If the amount of light 
entering the medium be called Iq, and the amount issuing from the medium I, we have 
ojIq — I, 
on the supposition that the extinction and the intensity of the light vary proportionally, 
when a represents the fraction of the original amount of light which has remained after 
passage through the medium. If the amount of light alo, which has passed through 
one medium, be now allowed to pass through a second whose coefficient is the amount 
of light issuing from the second medium will be from a third medium &c. 
The value of the result is not affected by any alteration in the order of the media, 
as such an alteration simply changes the order in which the coefficients &c. are 
placed. It is therefore quite immaterial, as regards the amount of light, in what order 
the various absorbing media are placed between the source of light and the instrument 
by which the effect is measured. 
In order to show that the extinction of the chemical rays is really proportional to the 
intensity of the hght, we have made the following experiments. 
