MEMOIR XVIII.] THE CHLOR-HYDROGEN PHOTOMETER. 259 



of the index on the scale at the end of a trial. It is to 

 be remembered that the cause of movement over the 

 scale arises from a condensation of hydrochloric acid, but 

 that condensation, though very rapid, is not instanta- 

 neous. Where time is valuable and the instrument in 

 perfect equilibrium, this condensation may be instanta- 

 neously effected by simply inclining the instrument so 

 that its liquid may pass down to the closed end #, but 

 not so much as to allow gas to escape into the other 

 side the inclination of the two sides to each other 

 makes this a very easy manipulation and the gas thus 

 brought into contact with an extensive liquid surface 

 yields up its hydrochloric acid in an instant. 



Directions for using the Photometer. Preliminary ad- 

 justment. Having transferred the liquid to the sealed 

 end of the siphon, and placed the cap on the sentient ex- 

 tremity, the voltaic battery being prepared, the operator 

 dips its polar wires into the cups^, </, which are in con- 

 nection with the wires a?, y. Decomposition immediately 

 takes place, chlorine and hydrogen rising through the 

 liquid and gradually depressing it, while, of course, a 

 corresponding elevation takes place in the other limb. 

 This operation is continued until the liquid has risen to 

 the zero. It takes but a few seconds for this to be ac- 

 complished. 



The polar wires having been disengaged, the photom- 

 eter is removed opposite a window, care being taken 

 that the light is not too strong. The cap is now lifted 

 off the sentient extremity a d, and immediately the liquid 

 ascends. This exposure is allowed to continue, and the 

 liquid suffered to rise as much as it will to the end a. 

 And now, if the gases have been properly adjusted, an 

 entire condensation will take place, the sentient tube a d 

 filling completely. In practice this precision is not how- 



