Wright and Larsen — Quartz as a Geologic Thermometer. 427 



fringence of quartz at different temperatures in sodium light, 

 taken while heating a quartz plate, are included ; in column 

 II, the measurements at different temperatures made while cool- 

 ing the same plate, are recorded ; in column III the readings 

 of birefringence in sodium light, taken both during heating 

 and cooling a second plate, are given, while in column IV the 

 results of birefringence measurements in lithium light, both on 

 heating and cooling a quartz plate, are included. 



The most satisfactory readings were made in sodium light 

 and there the inversion point is sharply marked. In the tables 

 the actual readings of birefringence have been corrected very 

 slightly to allow for change in thickness due to expansion, the 

 coefficients of expansion normal to the principal axis being 

 taken from the table of Le Chatelier.* As a matter of fact, 

 however, the differences between the recorded and corrected 

 values are very slight and might be disregarded without sensible 

 error. On one thick plate (2*89 mm ) the measurements of the 

 birefringence with the Babinet compensator in white light 

 were not satisfactory because of the disappearance of the dark 

 band at high temperatures, due probably to such a relation of 

 the rates of decrease of birefringence for different wave lengths 

 that the cold quartz of the Babinet could not compensate all 

 waves of the heated plates in precisely the same proportions, 

 and that therefore perfectly correct compensation was not to 

 be obtained. On thin plates this effect, which even in thick 

 plates is slight, was not appreciable. 



The curves of tig. 1 indicate that on heating, the birefrin- 

 gence of quartz decreases gradually but noticeably; at 575° 



*Loc. cit. The figures in the following table of expansion express the 

 expansion in mm. of a quartz rod 100 mm long ; in column I, the relative 

 expansion of quartz parallel to the principal axis c are listed and in column 

 II the observed expansion for a rod normal to e ; the values of the second 

 table were used in correcting for the change in thickness of the plate. 



Temp. 



I 



II 



270° 



0-20 



0-42 



480 



0-54 



0'84 



570 



0-93 



1-38 



660 



0-97 



1-59 



750 



0-95 



1-59 



910 



0-89 



1-57 



990 



086 



1-55 



1060 



0-86 



1-55 



Table 16. Birefringence measurements with Babinet compensator on 

 quartz plates at different temperatures in sodium and lithium lights. In 

 column I the measurements in sodium light taken while heating a quartz 

 plate are recorded : in column II, the readings taken while cooling the same 

 quartz plate are included ; in column III the birefringence measurements 

 made both during heating and cooling a second quartz plate are given ; while 

 in column IV the results of birefringence measurements in lithium light, 

 both on heating and cooling a quartz plate, are recorded. 



