8?0 



these needles the following valnes for the refractive indices : 



no = 1.600 ± 0.003 for vibrations parallel to the longer axis of 



a. 



the needles, and: n^ = \.b84: ± 0.002 for vibrations, perpendicular 

 to the first named. 



The question, whether the composing lamellae must be considered, 

 in agreement with Hautefeuillk and Margottet, as rhombic, is in 

 our mijid, very doubtful. As already the French author himself 

 suggested, it is nioi-e probable to suppose inonocUnlc sjmnietrv, and 

 a pseudo-trigonal aggiegation of these monoclinic individuals. Indeed, 

 by such a polysynthethic tw information, no \)seudo-tri(/o?ial, but a 

 pseu(\o-he.vago7ial aggregation woult result, because the original axes 

 of binary period would involve a symmetry-axis of the whole struc- 

 ture, having a period of 60°. 



The specific gravity of the modilication (1, 1898) being: {d^^o = 

 = 2,529, is in complete agreement with that of our biaxial metasi- 



licate, which had : ((74^ = 2.520 at 25° (.). 



Summarizing all these data, we must conclude from our experience 

 in this matter, that the so-called pseudo-trigonal modification of the 

 metasilicate can be no neiv modification of Li^SiO^, but only a 

 poly synthetic twinformation of the original biaxial form, imitating 

 verj' closely a true trigonal individual. 



This form with its apparent symmeiry, must in questions ofther- 

 modynamical equilibria be considered as the .same pha.se of the 

 compound, which is deposited from the molten mass ordinarily as 

 long, truly biaxial needles. In accordance with experience, brought 

 up to this date by the study of such mimetic crystals, it is just 

 quite clear, that no appreciable thermic effect, indicating some 

 noticeable, sudden difference in the total energy of the system, could 

 be detected in our experiments. 



§ 9. The lithiumhisilicate \ Lz^/S/jOj crystallizes from molten binary 



