On the Measurement of the Chemical Action of Light. 521 



fibres are found very much altered a few days after the extirpation of 

 the eye from the orbit, and they are almost destroyed twelve or 

 fifteen days after this extirpation, i. e. at a time where muscular 

 irritability is sometimes still existing. 



" Photo-chemical Researches. — Part I. On the Measurement of 

 the Chemical Action of Light." By Professor Bunsen of Heidel- 

 berg, and Henry Enfield Roscoe, B.A., Ph.D. 



The only instrument which has been applied to the measurement 

 of the chemical action of light was proposed in 1843 by Dr. Draper 

 of New York. The sensitive substance employed by him was a 

 mixture of chlorine and hydrogen, and by measuring the diminution 

 ensuing on exposure to light, he experimentally determined some 

 important relations of photo-chemical action. Draper's instrument 

 is, however, not adapted for accurate measurements, owing, in the 

 first place, to the fact that the gas is subject to varying pressure ; 

 and, in the second place, that the statical equihbrium, which must 

 exist between the free and dissolved gases, in order that the free gas 

 should consist of equal volumes of chlorine and hydrogen, was never 

 approached. 



In order to obtain more accurate results than was possible with 

 Draper's tithonometer, we sought for means of preparing a gas con- 

 taining equal volumes of chlorine and hydrogen ; this means was 

 found, notwithstanding Draper's contrary statement, in the electro- 

 lysis of strong aqueous hydrochloric acid. A series of volumetric 

 analyses proved that the gas thus evolved consisted, as soon as the 

 requisite saturation had been attained, of exactly equal volumes of its 

 component parts, and did not contain the slightest trace of oxygen 

 or oxides of chlorine. Another series of experiments with gas, simi- 

 larly prepared, but allowed to stand before analysis for many hours 

 in the dark in closed vessels, proved that, at the ordinary atmospheric 

 temperature, the gases do not enter into combination when the light 

 is excluded. Being thus enabled to prepare a substance which 

 undergoes decomposition on exposure to light, but does not change 

 on preservation in the dark, we proceeded to construct an apparatus 

 by means of which the laws of the chemical action of light might be 

 thoroughly investigated. After many fruitless attempts, we have 

 succeeded in constructing an instrument, by which not only accurate 

 comparative determinations can be made, but which has enabled us 

 to reduce the chemical action of light to an absolute measure. 



The most essential conditions fulfilled by our instrument are the 

 following : — 



1 . A continuous evolution of a gas consisting of exactly equal 

 volumes of chlorine and hydrogen free from all foreign impurities. 



2. Constant pressure on the gas and liquids throughout the appa- 

 ratus. 



3. Absence of all caoutchouc or other organic matter which might 

 alter the composition of the gas. 



4. Exclusion of all variation in the composition of the gas in the 

 apparatus from exposing the saturated liquids to the light. 



Phil. May. S. 4. No. 89. Huppl. Vol. 13. 3 N 



