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lysis of strong aqueous hydrochloric acid. A series of volumetric 

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

 requisite saturation hadl 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. 



5. Complete establishment of the statical equilibrium between the 

 free and dissolved gases. 



6. Elimination of the disturbing action of radiant heat. 



The instrument in which these conditions are fulfilled is con- 

 structed entirely of glass, and consists essentially of four parts, viz. 

 1, a decomposing tube in which the gases are generated from carbon 

 poles ; 2, a washing tube containing water, furnished with an air- 

 tight glass stopcock ; 3, the vessel in which the gases are exposed to 

 the action of the light attached to the other parts of the apparatus 

 by air-tight ground-glass joints ; and 4, a horizontal tube on which 

 the diminution of volume in the insolation vessel is observed by 

 means of a millimeter scale. 



