MEMOIR XVIII.] THE CHLOR-HYDROGEN PHOTOMETER. 255 



long and six tenths of an inch in diameter, blackened on 

 its interior, may be dropped over this sentient tube; it 

 being the office of the stage d to receive the lower end 

 of the cap when it is dropped on the tube so as to shut 

 out the light. 



The foot of the instrument, Jc I, is of brass ; it screws 

 into the block m, which may be made of hard wood or 

 ivory ; in this three holes, p, q, r, are made to serve as 

 mercury-cups ; they should be deep and of small diam- 

 eter, that the metal may not flow out when it inclines 

 for the purpose of transferring. A brass cylindrical cov- 

 er, L M, L M, may be put over the whole when it is de- 

 sirable to preserve it in total darkness. 



Things being thus arranged, the instrument is filled 

 with its fluid, prepared as will presently be described; 

 and as the tubes a Z>, b c are not parallel to each other, 

 but include an angle of a few degrees, in the same way 

 that lire's eudiometer is arranged, there is no difficulty 

 in transferring the liquid to the sealed side. Enough is 

 admitted to fill the sealed tube and the open one par- 

 tially, leaving an empty space to the top of the tube at 

 c of two and three quarter inches. 



Secondly, of the Fluid Part. The fluid from which 

 the mixture of chlorine and hydrogen is evolved, and by 

 which it is confined, is yellow commercial hydrochloric 

 acid, holding such a quantity of chlorine in solution that 

 it exerts no action on the mixed gases as they are pro- 

 duced. From the mode of its preparation it always con- 

 tains a certain quantity of chloride of platinum, which 

 gives it a deep golden color, a condition of considerable 

 incidental importance. 



When hydrochloric acid is decomposed by voltaic 

 electricity its chlorine is not evolved, but is taken up in 

 very large quantity and held in solution ; perhaps a bi- 

 chloride of hydrogen results. If through such a solution 



