441 



photograph was too pale. Tlie i-eyiilting state of' the heated apophjllite 

 remains tliiis tibsolutely fixed on cooling; after the data gi\en in 

 literatnre, the water expelled at 260° C. will be oidj resorbed after 

 about 3600 iionrs from an atmosphere of water vapour. Anticipating 

 on our experiments with the apophyllite of Iceland, we can remark 

 in this connection, that with this mineral, wliicli did not show the 

 five mentioned spots, there remained nothimj at all on the photo- 

 graphic pUite, after the crystal was heated, except some feeble action 

 on the places of the most intensive spols of the original image; 

 they only proved, that the transformation by the iieating was not 

 yet completely finished. In ]io case we have therefore succeeded in 

 proving, as before was done with the boracite, that the i)seudotetra- 

 gonal aggregation of monosym metric material, above a certain tem- 

 perature can be changed into (he really higher symmetrical form: 

 instead of such inversion, a change in the silicate-skeleton is {)ro- 

 duced, which at least during the short interval of the experiment 

 can be considered to be //-reversible, and which has nothing or not 

 directly to do with the real transformation into a true tetragonal 

 form . 



§ 9. In opposition to these American crystals, the used apophyllite 

 0Ï Iceland must be discerned as a most beautiful, glassy, and perfectly 

 clear crystal, which was determined to be a combination of sharp 

 pyramid {11 1| and basal pinacoid |001|. The angles of the pyramid 

 and of pyramid and basal pinacoid were variable within rather 

 wide limits; they deviated from the angles commonly mentioned in 

 literature by an amount of circa 30' (o 1° ; yet the reflected images 

 were splendid and quite sharp, this phenomenon too leading to the 

 supposition, that the tetragonal symmetry could only be a mimetic 

 one: 



|001| : illl| = 59°24' to 60°13; in literature : 60°32'. 

 jllll : llTli = - ; „ : 58°56' 



{111! : |111| = 74°38' to 75°39'; „ : 75° ; 



"rather oscillating". 



From this crystal two planparallel plates were cut, the one 

 parallel to |001|, the other to |]00|. 



The plate parallel to |001| between crossed nicols appeared 

 to be not completely isotropous, but to possess an extremely weak 

 birefringence, with the principal optical sections orientated perpen- 

 dicularly to the edges fUO) : (001). 



By means of a gypsum-lamella, giving the red of 1»* order, it 

 appeared to be divided into four sections, of which the diametrically 



