4.^9 



stresses, wliidi in (lioii- (iini are rauso<l hv an isoinorplioiis mixture 

 of optically positive and negative material '). The supposition of the 

 existence of siicli positive and negative apopliyllite-snhstances wliich 

 is reallv cdnlirmed in some cases by direct observation, must serv^e 

 at the same lime for the explanation of the very weak birefringence, 

 and the so-called leukocy elite-, and cJiroinocyclite-\)\\GnoinQ\vii. We will 

 demonstrate in the following pages, diat, — waiving the question, how 

 far the last mentioned phenomena need to be explained bv this 

 intergrowlh of optically positive and negative substances, -- in every 

 case the method of the RöNTGEN-radiation decides the (dternative 

 between the t/vo views indid>if(d)/f/ in fiivonr of Mallard's hi/pothesis. 



§ 8. The American apophyllites used were all transparent, pearl- 

 coloured crystals; they have a layer-structure parallel to jOOlj, to 

 which form also the direction of perfect cleavage is parallel. 



Without exception all these apophyllites are optically biaxial in 

 convergent polarised light, with positive character of the birefririqence. 

 The apparent axial angle is only small, with a dispersion: q<^v. 

 By means of a gypsumplate giving the red colour of 1''^ order, one 

 sees, that numerous blue-, and orange-tinged, rectangularly bounded, 

 very small tields are in juxta-, and superposition to each other, as 

 in a mosaic; the crystal makes the impression of consisting of an 

 innumerable quantity of perpendicularly very small lamellae, which 

 evidently are distributed and superposed in very unequal ji umber 

 and in a rather irregular way. 



All these preparations give, if tlie RöNTGENrays are directed per- 

 pendicularly to jOOlj, the radiograms, which in Table If are repro- 

 duced in the figures 1, 2, 3^' and 4. Of all these radiograms it is 

 again characteristic, that they possess a single plane of symmetry as 

 unique symmetry-element; it is placed in a vertical situation in all 

 reproduced tigures, and corresponds, as was found later, to a direction 

 perpendicular to the axial plane of the optically biaxial individuals. 

 The direction of this plane of symmetry can always rather easily 

 be fixed on the original negatives by the particular aggregation of 

 spots at the upper side of the image, which has the shape of a 

 double pinnacle between the two very distinct circular garlands of 

 spots there; and also by the fact, that it cuts symmetrically the 

 group of the five very intense black spots, which in fig. 1, 3 'and 4 

 are visible just beneath the centre: in tig. 2 these spots are invisible 



') The optical phenomena in basal sections of the optically positive apophyllites 

 Rie (after Kooke) exactly analogous to those which would be produced in thfi 

 originally uniaxial crystals, by stresses, working parallel to the edges (001); (110). 



29* 



