f)8 NINETEENTH REPORT. 



for the other two. These variations were, however, only in one case as 

 great as 0.7° C and usually less than 0.2.5' C, which may be considered 

 as being well within the limits of error. The uniaxiality curve is of 

 tlie same general character as the oiu- described in 191 I, althoujih the 

 imiaxiality temperatures for any given wave length are somewhat higher 

 tlian formerly recorded. Thus, we are able to indicate that the highest 

 uniaxiality temperature is about 91.3° C for 560|U,/x whicli corresponds 

 to 89.1 C for the same wave length of the earlier rei)ort. These new 

 observations are in fuller accord with those published by Hutchinson 

 and Tutton ■ and for reasons previously given are to be considered more 

 accurate than the earlier results. It will be noted from Plate II that 

 for light with the longer wave lengths, i. e. above 600/*/*, there is a 

 greater divergence in the observations than for light of shorter wave 

 lengths. This is due to the fact that the hyperbolic brushes of the 

 biaxial figures for light of the longer wave lengths are very broad and, 

 hence, it is more difficult to determine when the brushes are exactly 

 crossed thus forming the uniaxial figure for the wave length under con- 

 sideration. Some of the more important uniaxiality temperatures, as 

 determined from the curve, are as follows: 



t65fiM 89.02°C .589^M (D line) 91.12=C 



486 (F line) 89.90 6,56 (C line) 89.82 



'527 (E line) 91.02 670 89.47 



.560 91.27 687 (Bline) 89.02 



VARIATION OK THK ANGLK OF THE OPTIC AXES OX' GLATBERITE 



WITH TEMPERATURE. 



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

 with temperature were published for lithium and sodium lights. It was 

 clearly established at that time that the uniaxiality points for these two 

 tyi)es of liglit are considerably lower than had been previously determined 

 by Laspeyres\ In the present study, measurements of the variation in 

 the angle of the optic axes for different wave lengths were undertaken 

 employing the oil bath of larger capacity and the Fuess monochromator. 



The section was prepared from material from Villa Rubia, Spain, which 

 was not entirely homogeneous. This was clearly observable under the 

 miscroscoi)e. There were also sliglit variations to be noted in the observa- 

 tions made in ditferent ])arts of the section. Kozu' has recently called 

 attention to similar variations in certain feldspars studied by him. Our 

 observations are recorded in the following table: 



•Mineralogical Magazine, 1912, Vol. 1«, p. 257-63. 

 'Zeitschrift fuer Krystallofrrapliie, 1877, \ol. 1, p. ">2!i:>M). 

 MineraloKical Mafraziiu-. HMii. N'ol. 17, p. 2.17-.5:t. 



