. Carnegie Institution of Washington publication. 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 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, 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
. Carnegie Institution of Washington publication. 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 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, 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 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. FIG> 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
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