254 THE CHLOR-HYDROGEN PHOTOMETER. [MEMOIR XVII L 



closed at the end a. At d, a circular piece of metal an 

 inch in diameter, which may be called the stage, is 

 fastened on the tube, the distance from d to a being 

 2.9 inches. At the point x, which is two inches and a 

 quarter from d, two platinum wires, x and y, are fused 

 into the glass, and entering into the interior of the tube, 

 are destined to furnish the supply of chlorine and hy- 

 drogen ; from the stage d to the point Z, the inner bend 

 of the tube, is 2.6 inches, and from that point to the top 

 of the siphon c the distance is three inches and a half. 

 Through the glass at , three quarters of an inch from c, 

 a third platinum wire is passed ; this wire terminates in 

 the little mercury-cup r, and x and y in the cups p and 

 q respectively. 



A stout tube, six inches long and one tenth of an inch 

 interior diameter, ef, is fused on at c. Its lower end 

 opens into the main siphon tube; its upper end is turned 

 overMit/, and is narrowed to a fine termination so as 

 barely to admit a pin, but is not closed. This serves 

 to keep out dust, and in case of a little acid passing out, 

 it does not flow over the scale and deface the divisions. 

 At the back of this tube a scale is placed, divided into 

 tenths of an inch, beins; numbered from above down- 



/ C 



wards. Fifty of these divisions are as many as will be 

 required. Fig. 2 shows the termination of the narrow 

 tube bent over the scale. 



From a point one fourth of an inch above the stage <7, 

 downwards beyond the bend, and to within half an inch 

 of the wire z, the whole tube is carefully painted with 

 India-ink, so as to allow no light to pass; but all the 

 space from a fourth of an inch above the stage d to the 

 top of the tube a is kept as clear and transparent as 

 possible. This portion constitutes the sentient part of 

 the instrument. A light metallic or pasteboard cap, A D, 

 closed at the top and open at the bottom, three inches 



