79H 



]\v a semi-evliiidrical sii|)|X)rt, attached to the horizontal part of the 

 brass |)late, it is borne up from beneath, while a copperwire, wrapped 

 i-oiiiid the double-walled cylinder of the furnace, helps to keep the 

 apparatus in its position. It is necessary- of course, to adjust the 

 horizontal axis of the furnace exactly in the same level as the small 

 canal for the emerging rays. The whole arrangement of the Röntgen- 

 tube, the lead screen, etc., corresponds principally with that described 

 formerly by one of us.^) 



\N 3. 7'ke iimteridl. The boracite, used in this investigation, was 

 from Sehnde, in Hannover. It crystallized in clear, pale blue-green 

 large crystals, showing the form |110|. Two planeparallel plates were 

 cut from a crystal, one perpendicular to a binary, the other one 

 perpendicular to a ternary axis of the apparent regular form. 



Oui' experiments were only made with a plate, cut perpendicular 

 to a binary axis; at the same time such a plate must be perpendi- 

 cular to a quaternary axis of the Bravais' space-lattice, if the crystal 

 is really of the regular system. 



By microscopic investigation the strong birefrijigence of the crystal- 

 plate was easily demonstrated. It showed a typical polysynthetic 

 structure ; between crossed nicols it was in uo position totally dark, 

 hut only locally. The composing lamellae showed high interference- 

 colours ; in convergent j)olarized light an interference-image of a biaxial 

 crystal, almost perpendicular to an optical axis, was visible, with a 

 dispersion, which would be in agreement with rhombic symmetiy. 



When the crystal-plate was heated in the microscope-furnace, al- 

 ready described by one of us, — in which furnace the crystal rests 

 on the hot junction of the used thermoelement, — the polarisation- 

 colours between crossed nicols change gradually from violet to yellow, 

 blue and grey; then the held of the microscope gets dark suddenly 

 at 266° C. On cooling the birefringence returns as suddenly as it 

 disappeared; it is an extremely remarkable fact, that^ahiiost the same 

 lamellae return on that occasion, ivhich were present already before 

 heating. 



The experiment can be repeated arbitrarily ; thus we are quite 

 sure, that in our experiments at fully 300° C. the optically isotropous 

 form has always been present, while the birefrinyent one must have 

 returned always after cooling to room-temperature. 



§ 4. Our experiments now were made in such a way, that first 

 a RöNTGEN-photograph was taken, when the crystal-plate was out 



1) H. Haga, Ann. d. Physik (4). 23. 439, 440. (1907). 



