174 HYDRATES IN AQUEOUS SOLUTION. 



of the slit of the spectre-graph. The lower end of F moved over a scale 

 parallel to the slit, and in the plane of the jaws of the latter. 



The flange at the bottom of the framework was made of brass, only 0.014 

 cm. thick, so that the absorbing liquid might be as near the slit as possible. 



(2) An accurately ground, plane parallel plate of quartz, 40 mm. long, 

 18.5 mm. wide, and 2 mm. thick, had cemented to the periphery four rectan- 

 gular sheets of glass 8 mm. high. Hence the greatest depth of liquid which 

 could be studied by the aid of this cell was 6 mm. 



(3) In fig. 68, a, b,c, and d designate the vertices of a section of the quartz 

 plate, made by a plane perpendicular to the plane whose trace is the line 



ad; ab was 2 mm., ad was 34.8 mm., and the 

 angle between the planes of ad and be was 55" 

 of arc. The horizontal width of the wedge 

 was 10 mm. Glass walls surrounded three 

 sides of the wedge, as the outline indicates. 

 F IG> 68. The reason for using the wedge was to coun- 



teract the deviation and dispersion produced 



by the solution in the cell. The angle of the liquid wedge could be varied 

 until the deviation effected by the quartz wedge nullified the average action 

 of the absorbing solution. At first it was supposed that with liquid wedges 

 of 15 or so minutes of 



' 



arc, a plane parallel to 



o o 



/ 

 the quartz plate could 



be used successfully instead of 

 a quartz wedge. Actual tests, 

 however, showed that a plane i 



I A 



parallel plate could not be re- p"| 



lied upon, in general, to give 



correct results. Finally, the 



, ., , FIG. 69. 



quartz wedge was made with 



the utmost care by an expert optician, special pains being taken to have the 

 edge through D perpendicular to the plane a, b, c, d as sharply defined as 

 possible, and the surfaces whose traces are noted by ad and be were accu- 

 rately plane. 



(4) Figure 69 represents a side view and an end view of the vulcanite frame, 

 into which the box just described fitted. This frame was shaped out of 

 a single block of vulcanite, since experience showed that a cemented system 

 of several pieces was not desirable; also a dielectric was needed to keep the 

 sparks from jumping to the screws. P indicates a little depression which 

 fitted over the point of the screw T. P' designates the end of a straight 

 line along which the rounded extremity of the screw T' slid. P" is the 

 cross-section of a shallow V-shaped groove along which the pointed end 



