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XVII. Photo-chemical Researches. — Part I. Measiireinent of the Chemical Action of Light. 
By Egbert Bunsen, Professor of Chmiistry at the University of Heidelberg, and 
Henry Enfield Roscoe, B.A., Ph.B., Professor of Chemistry at Owens College, 
Manchester. Ccmimumcated by Professor Stokes, Sec. R.S. 
Eeceived November 12, — Bead November 20 and 27, 1856. 
The first and only attempt which has been made to refer the chemical action of light 
to a standard measure, is to be found in the researches of Draper. The description of 
the instrument and mode of observation employed by him was pubhshed in 1843, in 
volume xxiii. p. 401 of the ‘ London, Edinburgh and Dublin Philosophical Magazine.’ 
Even with this instrument, which as we shall show is in many respects very defective. 
Draper has succeeded in establishing experimentally some of the most important rela- 
tions of the chemical action of light. 
In these experiments Draper collected hydrogen, evolved by electrolysis, over hydro- 
chloric acid saturated with chlorine, and to this hydrogen he admitted so much chlorine, 
either by diffusion from the saturated acid or by electrolysis, that the mixture consisted 
of nearly equal volumes of the two gases, and entirely, or almost entirely, disappeared on 
exposure to light. The alteration in the volume of the gaseous mixture arising from 
the absorption of the hydi’ochloric acid formed by the action of the light, was read off 
on a scale, and being, within certain limits, proportional to the time of exposure, 
served as a measure of the chemical rays. -r. 
This method affords, however, no means for correctly determining the point at which 
the gas collected over the acid consists of equal volumes of its constituent parts ; still more, 
however, was the accuracy of Draper’s instrument impaired by not considering certain 
conditions, which must be fulfilled, if exact photo-chemical measurements with chlorine 
and hydi'ogen are required. The first and most essential condition, as influencing the regu- 
larity of the measurements, relates to the perfectly constant composition of the gas. 
MTien two gases, V and V„ having respectively the coefficients of absorption a and «„ 
are mixed together in such proportions that the unit of volume contains v volumes of 
the one gas and volumes of the other, and when such a mixture passes through a 
liquid, it rapidly changes its composition, according to a complicated law, until the 
moment arrives when the gases absorbed by the liquid are contained in the relation of 
av to If the gaseous mixture be collected over the liquid before the relation ■— 
has been estabhshed, an exchange between the dissolved and free gases must occur, and 
hence the composition of the gaseous mix ture must constantly alter, the variations being 
3 A 2 
