ARTIFICIAL PRECIOUS STONES—HEATON. 221 
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- 
laving 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 IT. 
TaBLeE II.—lIdentification of imitation precious stones. 
Paste. Stones. 
Index of refraction rarely exceeds 1.65. Index of refraction ranging up to 2.4. 
Single refracting, or false double refracting owing to | Double refracting, with exception of diamond, 
strain. garnet, and spinel. 
Never pleochroic. Often strongly pleochroic when colored. 
Hardness always below 7. ; Hardness 7 or over (with a few exceptions). 
Specific gravity usually above 4. Specific gravity usually below 4. 
Thermal conductivity low. Thermal conductivity comparatively high. 
Opaque to X rays. Translucent or transparent to X rays. 
Generally show spherical bubbles and curved strie. | Frequently show lamination or inclusions. 
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. 
