96 NINETEENTH KEPORT. 



In 1913 observations on the change of the optic angle of glauberite 

 were published but for only two types of monochromatic light, lithium 

 and sodium. In the present paper new observations are given on the 

 variation in the size of the optic angle of glauberite for light of several 

 different wave lengths and on the temperatures for uniaxiality for wave 

 lengths between 511/a/ji and 687/u,/u,. 



APPARATUS. 



The apparatus used in these new experiments consisted of the axial 

 angle apparatus and the large metallic oil bath of 150 c.c. capacity 

 employed in the original experiments in 1911. This bath was. however, 

 lieated by a larger resistance coil than that employed in the experiments 

 reported in the second paper. The heating apparatus consists essen- 

 tially of a tile plate about 20 cm. square on the under side of which are 

 fastened a series of small resistance coils. The oil bath, containing 

 the section to be investigated and into which the bulbs of the two long 

 thermometers penetrate, and the heating plate are all enclosed in an 

 asbestos box, the sides of which are easily removable. Experience has 

 shown that more accurate and consistent results are obtained when a 

 larger oil bath is employed than when one of smaller capacity such as 

 was described in the second paper is used. An oil bath of comparatively 

 large capacity permits a more gradual increase or decrease of tempera- 

 ture to take place as also an easier maintenance of a constant tempera- 

 ture. These factors are highly desirable inasmuch as gypsum responds 

 more readily to slight changes in temperature than does the mercury in 

 the thermometers. By placing several secondary resistances in tin- 

 circuit, the temperature within the oil bath could be very easily regulated. 

 The thermometers permitted readings to be made to 0.1° C. The results 

 given in the various tables have all been corrected and take into con-" 

 sideration the temperature of the room surrounding the apparatus, the 

 temperature in the asbestos box surrounding the oil bath, and the length 

 of the thermometer tubes. Curves have been developed from such cor- 

 rected observations. 



Monochromatic light was obtained from a Fuess monochromator using 

 as the original source of light a small hand-feed, direct current arc 

 light. The monochromator was first carefully adjusted for sodium and 

 lithium lights so as to insure accurate readings. This instrument is ex- 

 ceptionally useful in the determination of uniaxiality temperatures 

 as a simple rotation of tlie drum allows exceedingly rapid changes 

 in the character of the light supplied to be made. In fact, an apparatus 

 of this type is indispensable for observations involving the use of 



