PHOTOCHEMICAL RESEARCHES. 63 



ployed by Draper for evolving the sensitive gas never could have furnished 

 it of constant composition. 



Draper's instrument consists of a siphon tube, of which one limb is short 

 and closed, and the other longer, narrow and open at top. The long limb is 

 furnished with a scale, the shorter one has two platinum wires melted into 

 the glass near the bend. The whole of the short, and part of the long limb, 

 is filled with hydrochloric acid saturated with chlorine, and by means of an 

 electric current the acid can be decomposed and the gases collected in the 

 short limb. According to Draper no gaseous chlorine is evolved during the 

 electrolysis of hydrochloric acid ; the hydrogen, however, set free at the 

 negative pole passing through the liquid displaces some of the chlorine held 

 in solution, and thus a sensitive gas is obtained and collected in the shorter 

 siphon limb. The composition of this gas cannot, however, be constant, for 

 according to the law of gas absorption, when a mixture of gases is collected 

 over water, the free gas cannot possess a fixed composition before a certain 

 relation between the volumes of the dissolved gases has been attained. Until 

 this equilibrium has ensued, a continuous interchange between the volumes 

 of free and dissolved gases must take place, and in the case of the tithono- 

 meter this equilibrium is not even approached. Another more considerable 

 source of error in Draper's instrument lies in the difference of pressure to 

 which the gas is subjected during the experiments, arising from the gradual 

 fall of the liquid in the longer limb in proportion as the sensitive gas is 

 acted upon by the light. 



Having assured ourselves that the indications of the tithonometer cannot be 

 relied on, the necessity of obtaining an instrument in which the foregoing 

 and many other essential conditions are fulfilled, became apparent. The 

 first object therefore was to obtain a gas consisting of equal volumes of 

 chlorine and hydrogen of constant composition. This object we attained 

 (contrary to Draper's express statement) by the electrolysis of aqueous 

 hydrochloric acid. Exact volumetric analysis convinced us that as soon as 

 the acid is saturated with the two gases, in accordance with the laws of ab- 

 sorption, the evolved gas consists exactly of equal volumes of chlorine and 

 hydrogen, unaccompanied by oxides of chlorine, or hydrogen or other im- 

 purities. After many fruitless attempts, we have at length constructed an 

 apparatus in which the second, and all other required conditions are satisfied, 

 and by means of which we have been enabled not only to obtain a relative, 

 but even an absolute measure for the chemical action of light. 



This apparatus, represented in PI. III., is constructed entirely of glass, 

 and consists essentially of four parts : firstly, a tube (a) containing carbon 

 or platinum poles i'astened on platinum wires melted through the glass, 

 serving for the electrolytic decomposition of the aqueous hydrochloric acid ; 

 secondly, a set of bulbs for washing the gas, furnished with a glass stopcock 

 for shutting off the supply of gas; thirdly, a small flattened glass bulb (c) 

 containing water, in wliich the gas is exposed to the action of the light; and 

 fourthly, a capillary tube (d) furnished with a millimetre scale, on which the 

 diminution of volume caused by the absorption of the hydrochloric acid is 

 accurately observed by the advancing column of water. Each of these 

 pieces is fitted air-tight into its place by ground-glass joints, so that no 

 caoutchouc or other organic substance comes in contact with the sensitive gas. 



In this arrangement the pressure is rendered constant throughout the 

 whole apparatus by raising or depressing the exit tube dipping into the 

 bottle (e) filled with water, and by means of the horizontal absorption tube 

 (d) the pressures before and after the experiment do not differ by two milli- 

 metres of water. 



