SOME NEW THERMO-OPTICAL OBSERVATIONS ON 

 GYPSUM AND GLAUBEHITE. 



BY E. H. KRAUS AND ALHERT li. PE( K. 

 INTRODUCTION. 



Several years ago a number of articles were published from this 

 laborator}' dealing with the changes in the size of the angle of the optic 

 axes with tem{)erature and also with the determination of the tempera- 

 ture of uniaxiality of gypsum^ and glauberite'. 



In the first paper dealing with gypsum a comparatively large oil bath 

 was used, the source of heat being two Bunsen burners. While the 

 apparatus was somewhat crude in character, it nevertheless permitted 

 extensive observations to be made with a considerable degree of accuracy 

 on the variation in the size of the optic angle with temperatures between 

 18.2" C and 132.5^ C. It was definitely determined that uniaxiality 

 for sodium liglit, the only type of monocliromatic light employed, occurs 

 in the neighborhood of 90° C. Observations were also made by heating- 

 gypsum in an air bath and it was shown that the greatly divergent 

 results concerning uniaxiality temjaeratures obtained by previous investi- 

 gators were due to too rapid heating and the use of air baths. In the 

 second article the study of the uniaxiality temperatures for gypsum 

 for a large number of wave lengtlis was made employing a new heating- 

 apparatus consisting of a small glass container for the oil which was 

 heated by an electric resistance coil. Light of various Avave lengths 

 was obtained by the use of a Fuess monochromator. The curve showing 

 tlie temperatures of uniaxiality indicated clearly that the maxjmum 

 temperature for uniaxiality occurs when light having a wave length of 

 about 560fifi is used. In making the observations for gypsum recorded 

 in these two papers, plates prepared by Steeg und Renter, Homburg 

 V. d. Hohe, were employed. Unfortunately the localities from whieli 

 the material was obtained were not known. It was therefore thouglit 

 advisable to repeat the second investigation using sections from several 

 known localities. The first portion of this paper, therefore, gives some 

 new observations on the uniaxiality temperatures of gypsum from Ells- 

 worth, Ohio; Eisleben, Germany; and (yianciana, Sicily. 



I9th Mich. Acad. Sci. Rept., ini7. 



'N'eues Jalirhucli fuer Mineralogrie, (Jeiilogio mid PalaeontoIoKie, 1912, Vol. 1, p. 123-tfi, 

 and Centralhlatt fucr Miiieralojrie, (ieoloarie, und PalaeontoloKic, 191 1, p. 35G-9. 

 -Zcitschrift fiicr Krystallographic, I ins. \'ol. ,52, p. .32l-(i. 



