254 THE CHLOR-HYDROGEN PHOTOMETER. [MEMOIR XVIII. 



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 #?, 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 3, three quarters of an inch from c, 

 a third platinum wire is passed ; this wire terminates in 

 the little mercury-cup ?', 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 

 over at/, 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, being numbered from above down- 

 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 J, 

 downwards beyond the bend, and to within half an inch 

 of the wire 2, 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 



