RECENT PROGRESS IN CRYSTALLOGRAPHY 103 



point, through which a pressure is applied under the centre 

 of the plate equal to the weight suspended from the other 

 end ; and a delicate control apparatus, shown in fig. 6, which 

 only permits the weight to operate with extreme slowness, 

 so that the bands may be readily counted as they pass the 

 reference mark. The elasmometer also carries its own inter- 

 ference tripod in rigid connection with the knife-edges, for the 

 support of the glass disc, and the second reflecting surface is 

 formed by a black glass disc at the T top of a vertically sliding 



Fig. 6. 



aluminium rod, whose pointed lower end rests upon the centre 

 of the substance plate, and consequently moves with the latter. 

 These parts are more clearly shown in fig. 7. This instrument 

 has been fully tested in a research just concluded on the 

 elastic constants of pyrites, and proves as fully satisfactory as 

 the dilatometer, so that it may be employed with confidence 

 in future work on the sulphates. 



The Results of the Investigations 



As regards the external morphology of the crystals it has 

 been conclusively demonstrated that each salt possesses its 



