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MEMOIR XVJII.] THE CHLOR-HYDROGEN PHOTOMETRIC 



The following are the dimensions of the 

 use. The platinum plate is half an inch wide 

 inches long; it dips into a cylinder of porous bis 

 ware of the same dimensions, which contains nitric 

 Outside this porous vessel is the zinc, which is a 

 der one inch in diameter, two inches long, and two tenths 

 thick; it is amalgamated. The whole is contained in a 

 cup, two inches in diameter and two deep, which also re- 

 ceives the dilute sulphuric acid. 



The force of this battery is abundantly sufficient both 

 for preparing the fluid originally and for carrying on 

 the photometric operations. It can decompose hydro- 

 chloric acid with rapidity, and will last with ordinary 

 care a long time. 



Before passing to the mode of using this photometer, 

 it is absolutely necessary to understand certain theoret- 

 ical conditions of its equilibrium. These in the next 

 place I shall describe. 



Theoretical Conditions of Equilibrium. This photom- 

 eter depends for its sensitiveness on the exact proportion 

 of the mixed gases. If either one or the other is in ex- 

 cess a great diminution of delicacy is the result. The 

 comparison of its indications at different times depends 

 on the certainty of evolving the gases in exact, or, at all 

 events, known proportions. 



Whatever, therefore, affects the constitution of the sen- 

 tient gases alters at the same time their indications. Be- 

 tween those gases and the fluid that confines them cer- 

 tain relations subsist the nature of which can be easily 

 traced. Thus, if we had equal measures of chlorine and 

 hydrogen, and the liquid not saturated with the former, 

 it would be impossible to keep them without change, for 

 by degrees a portion of chlorine would be dissolved and 

 an excess of hydrogen remain ; or if the liquid was over- 



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