793 



a polysyntlietic twinformalion of six hemimoi-phic i-liomhic ci-yslals. 

 whose twiniiiiio-plaiies are those of tlio appareiil rhomheiidodecahechoii. 

 ï^orraal to every plane of this I'onii, the iiilerfereriee-iina^e of a 

 biaxial crystal can he observed in converi-ent polarised li,i>-ht, the 

 plane of the optical axes being- parallel to the longer diagonal of 

 each face of the psendo-rhombendodecahe(h'on. At tcniperatnres between 

 260° and 280° C., the boracite snddenly becomes optically isotropons; 

 then it has got perfectly i-egnlar, and its optical properties ai-e now 

 in complete agreement with its extei-nal form. On cooling, the 

 original birefringence returns as snddenly, as it has gone : the crystal 

 represents afterwards again the case of an optically anomalons one. 

 These general considerations will he sufficieni here for our juir- 

 pose ; therefore we will now begin the description of oni- cxpei-iments. 



§ 2. Tke heaümi-apjiaratiLs. To fuHil the condition, that the 

 RöNTGEN-rays might be transmitted as well at higher as at lower 

 temperatnres, a fnrnace of the form described here in detail, was 

 constrncted. 



A box with donble walls was made of polished l)rass ; it 

 incloses the whole fnrnace like a screen, and is kept, at a constant 

 temperature by means of a circulating stream of cold water. The 

 hollow box is composed of two se[)arate parts : one of them C^, 

 bears a tube R, which can be connected wnth the IvihNTGEN-apparatus; 

 further it has a cylindrical hole z, serving as a canal for the thin 

 bundle of RoNTGEN-rays. Cold water enters the box at / ; after cir- 

 culation it goes by V and B to the hollow cover Q.^, and leaves 

 the apparatus at U. The cover Q^ is fixed in position on Q^ by 

 means of three equidistant screws *S'. Q^ possesses at /' an oblicpie 

 perforation, which serves for the adjustment of the thermoelement 77/. 



The heating-coil I) consists of platinum-wire, 0,4 m. in. in diameter ; 

 it enters the furnace at P, where it is insulated from the brass box 

 by means of a porcelain-tube, and leaves the apparatus in the same 

 way by a second hole of this kind. The heating coil is wound 

 round a core of copper A ^), from which it is insulated by means 

 of a thin layer of asbestos-paper ; the coil needs to be applied oidy 

 in a single layer. The metal core A is held in a central position 

 by means of six pieces of carbon ; all intervening space is filled 

 up with disks of heavy asbestos, cut in suitable dimensions. 



-) For higher temperatures it must be recommended, to prepare this central 

 part of the furnace from nickel, or to coat it heavily willi gold ; for the highest 

 temperatures (up to IGOO (^\),alandum-covesoï{\ïel^ortonCompuriij\n Worct'sttr 

 (Mass.) U. S. A., are an excelleiit material. 



51* 



