260 THE CHLOR-HYDROGEN PHOTOMETER. [MEMOIR XVIII. 



ever obtained, and if a bubble as large as a pepper-corn 

 be left, the operator will be abundantly satisfied with 

 the sensitiveness of his instrument. Commonly, at first, 

 a large residue of hydrogen gas, occupying perhaps an 

 inch or more, will be left. It is to be understood that 

 even this large surplus will disappear in a few hours by 

 absorbing chlorine. But this is not to be waited for ; 

 as soon as no further rise takes place, in a minute or two, 

 the siphon is to be inclined to one side, and the residue 

 turned into the open side. 



Now, recurring to what has been said on the equilib- 

 rium, it is plain that this excess of hydrogen arises from 

 a want of chlorine in the photometric liquid. A proper 

 quantity must therefore be furnished by proceeding as 

 follows : 



The sentient tube being filled with the liquid by in- 

 clination, connect the polar wires with p, q, as before. 

 These may be called generating wires. Allow the liquid 

 to rise in b c until the third platinum wire 2, which may 

 be called the adjusting wire, is covered an eighth of 

 an inch deep. Then remove the negative wire from the 

 cup p into the cup r, and now the conditions for saturat- 

 ing the liquid are complete ; hydrogen escaping from 

 the surface of the liquid at 2, and chlorine continually 

 accumulating and dissolving between x and d. This 

 having been carried on for a short time, the gas in a d 

 is to be turned out by inclination and the instrument 

 recharged. That a proper quantity is evolved is easily 

 ascertained by allowing total condensation to take place, 

 and observing that only a small bubble is left at a. 



It will occasionally happen in this preliminary adjust- 

 ment that an excess of chlorine may arise from continu- 

 ing the process too long. This is easily discovered by 

 its greenish-yellow tint, and is to be removed by inclin- 

 ing the instrument and turning it out. 



