Aqueous Solutions of Hydrogen Chloride, &c. 309 



at the same temperature. In order that the prism might be in contact 

 with the water-jacket it was necessary that this should be carried by 

 the table of the goniometer, and therefore that it should be as small 

 and light as possible. 



A slow stream of water from a tap was allowed to flow through a 

 coil of about 5 metres of " compo " pipe, placed in a metal water-bath 

 supported over a small gas flame ; the temperature of the water being 

 kept constant by means of a Harcourt gas regulator. From the coil 

 the water passed through a rubber tube to the water-jacket. This 

 consisted of two flat brass boxes 1*5 cm. deep and 12 cm. in diameter, 

 between which the prism was placed ; the lower box being circular, the 

 upper annular. The boxes were fixed together concentrically and 

 their interiors connected by two vertical brass tubes 5 -2 cm. long. 

 The brass plate which formed the top of the lower box was 5 mm. 

 thick and 15 cm. in diameter; the levelling screws, by which the 

 water-jacket was supported on the table of the goniometer, worked 

 in holes drilled in the overhanging edge of the plate, and the prism 

 rested on it. 



A brass ring with a screw cut on its inner surface was fixed round 

 the top of the central opening in the upper box, a brass tube 2 '5 cm. in 

 internal diameter, with a screw cut on its outer surface, worked into 

 the thread on the ring, and by screwing this tube down on to the top 

 of the prism, the prism could be held in position. The object of this 

 particular arrangement was to enable the prism to be filled and 

 emptied by means of a pipette, without interfering with its position, 

 and to allow a thermometer, and a platinum stirring wire, to be in- 

 troduced into the liquid whose refractive index was to be observed. 



The third face of the prism was in actual contact with the flat sur- 

 face of one of the vertical tubes, by means of which the two boxes 

 were connected together. The cross section of these two tubes was 

 such, that the prism was, as far as possible, surrounded by the water- 

 jacket. Paper tubes, fitted over the ends of the collimator and tele- 

 scope, projected into the openings in the water-jacket. By packing 

 the spaces between these paper tubes and the water-jacket with cotton 

 wool, the sides of the prism were, to a considerable extent, shielded 

 from air currents, without preventing the adjustment of the prism and 

 telescopes. 



Two small metal pipes were fixed to the top of the upper cylindrical 

 box, the one ending just within the box, the other reaching down 

 through one of the connecting tubes already mentioned, nearly to the 

 bottom of the lower box. To these metal pipes rubber tubes were 

 attached ; through the one the water from the coil flowed into the 

 jacket, through the other from the jacket to the waste. 



By keeping the temperature of the water-bath at about 20 (the 

 temperature necessary varied with the temperature of the room, an 



2 B 2 



