ARTIFICIAL, PRECIOUS STONES HEATON. 



~ -J JL 



instead of lime as the basic constituent, the refractive index and 

 dispersive power of glass are much increased, and by replacing the 

 alkaline constituent by thallium oxide in the same manner the 

 refractive index may be raised as high as 1.96 and the dispersion to 

 0.049. 1 By adjusting the composition in this way, and preparing the 

 glass with the greatest regard to the purity of the materials, manipu- 

 lating it, moreover, in a similar elaborate manner to that employed 

 in the production of glass for optical instruments, in order to secure 

 the utmost freedom from striation and inclusions, it is possible to 

 imitate any precious stone accurately, as far as outward appearance 

 is concerned. 



The trouble is, however, that with glass the more you increase its 

 refractive power in this way the softer and less durable it becomes, 

 until you find that the very "dense" flint used for the refractometer, 

 having a refractive index of 1.8049, is so soft that it has to be handled 

 with great care to avoid scratches, and so little resistant to decay 

 that in a comparatively short time the exposed surface becomes 

 corroded, which is the one weak point of this instrument. It is true 

 that this softness may be counteracted to some extent by further 

 adjustment of the composition, adding a proportion of alumina and 

 zinc, and by careful thermal treatment of the finished stone in some 

 such manner as that originally introduced by Bastie, in which the 

 glass is case-hardened by plunging whilst hot into a bath of oil. In 

 some of the best modern paste I have found a refractive index of 

 over 1.6 combined with a hardness close on that of quartz, but this 

 is the absolute limit, and it is not possible in any way to obtain 

 a paste that can not be scratched with a hardened steel point. Paste 

 can also be readily identified by means of the scientific tests, as indi- 

 cated in Table II. 



Table II. — Identification of imitation precious stones. 



1 These are the constants given for the Jena glass, No. S. 57; the specific gravity is 6.33. Refractive index 

 of diamond is 2.4, and dispersion 0.057. 



