Cadmium, and Mercury. 345 



shows any lagging of one curve behind the other to indicate 

 any absorption or evolution of heat in either substance. 



The temperature interval which is included by the curves 

 will be found by comparison with the sequel to lie above the 

 true inversion point. It was found, however, by actual trials 

 that the transformation was subject to hysteresis and, on com- 

 paratively rapid heating, occurred in this interval. The reac- 

 tion sphalerite < > wurtzite was proved to be reversible by 

 long heating at selected temperatures (see below), and conse- 

 quently there must be a heat absorption when sphalerite is 

 heated ; and further, since the change was found to be com- 

 plete inside of a time interval of six minutes and a temperature 

 interval of 70°, this effect must be quite small. The inversion 

 point was determined with considerable accuracy by long heats 

 at measured temperatures, after which the direction of the 

 change was determined microscopically. The sluggish nature 

 of the change from wurtzite to sphalerite made it possible to 

 cool the former to room temperature without any transforma- 

 tion into sphalerite. By continually narrowing down the 

 temperature interval on either side of which the transformation 

 was reversed, the inversion point was determined within ± 5°. 



The apparatus* used for most of the work consisted of a 

 tube of Berlin porcelain 25 mm inside diameter and 500 mm long, 

 closed at one end and glazed inside and out. The tube, which 

 was held in a vertical position, was closed at the top by a doubly 

 perforated graphite cover. Through the central orifice passed 

 a glazed Marquardt tube of 6 mra internal diameter, also closed 

 at one end, which protected the thermoelement. Attached to 

 the closed end were one or more short tubes, also closed at one 

 end, of the same size and material, which contained the zinc 

 sulphide in the form of coarse powder. The large enclosing 

 tube was traversed by a current of dry hydrogen sulphide, 

 which entered by a second Marquardt tube open at both ends. 

 The heating was done by a resistance furnace which surrounded 

 the cylinder. 



The thermocouple which was used for the final temperature 

 measurements was calibrated after they were finished, at the 

 silver point (960°). 



By reference to Table II it will be seen that the inversion 

 point of zinc sulphide may be located between 1015° and 1024°, 

 i. e., 1020° ± 5°. As was mentioned above, the reversion of 

 wurtzite to sphalerite is very slow. Instead of heating in an 

 atmosphere of hydrogen sulphide, it was naturally more con- 

 venient in long continuous heats to use a vacuum apparatus.! 



* Allen, Crenshaw and Johnston, this Journal, xxxiii, 195, 1912. 

 flbid., xxxiii, 210, 1912. 



