30 ISOMORPHISM AND THERMAL PROPERTIES OF FELDSPARS. 



The borax glass upon which our measurements were made was 

 prepared in the usual way by heating the crystals until the water of 

 crystallization had been driven off and the viscous mass was reason- 

 ably free from bubbles. If the borax is pure, the anhydrous product , 

 when cooled, is a brilliant, colorless glass, isotrophic, of conchoidal 

 fracture, and specific gravity 2.37. The specific gravity was deter- 

 mined in the fraction of kerosene boiling above 185 C. About 100 g. 

 of this glass were then broken up and placed in a platinum crucible 

 in the electric furnace. The thermo-element was placed in position 

 as indicated in fig. 3, the heating current properly regulated, and ob- 

 servations of the temperature made at intervals of one minute, while 

 the glass softened and passed gradually over into a thin liquid (8oo). 

 Then the current was reduced and the cooling curve observed in the 

 same way. These observations gave an unbroken curve, both for the 

 heating and cooling, as in the case of all the glasses,* without a definite 

 melting or solidifying point, although the arrangements for detecting 

 an absorption or release of heat were very sensitive. Prodding at in- 

 tervals with a platinum rod showed the change to be perfectly gradual 

 from a clear, hard cake through all degrees of viscosity to a fairly thin 

 liquid and back again. This observation is of considerable interest 

 as showing that the absence of bounding phenomena between the cold 

 glass, which fulfills the mechanical conditions for a solid very perfectly, 

 and the liquid, is not confined to mixtures of complicated chemical 

 composition, but is exhibited also by true chemical compounds of 

 undoubted purity. It is, therefore, not conditioned by composition, 

 but by the physical nature of the substance. 



Having verified this behavior of anhydrous borax by several repeti- 

 tions of the experiment, various disturbing influences were applied to 

 the slowly cooling liquid in the hope that some temperature or range 

 of temperature would be found within which the vitreous condition 

 would prove unstable and crystallization be precipitated. The jar 

 produced by an electric hammer pounding upon the outside of the 

 furnace during cooling proved to be sufficient to bring down the entire 

 charge as a beautiful crystalline mass of radial, fibrous structure, 

 brilliant luster, rather high refractive index, and increased volume. 

 Fig. 4 will give a good idea of the appearance of the anhydrous crys- 

 talline borax in the crucible. Its specific gravity proved to be 2.28 

 as compared with 2.37 for the glass, a somewhat unusual relation, f 

 which may, in part, account for the quasi stability of the vitreous form 

 during cooling. 



* See Tammann, loc. cit.; also Roozeboom, "Die heterogenen Gleichgewichte, 

 etc.," Braunschweig, 1901. 



+ Tammann, loc. cit., p. 47 et seq. 



