Day and Shepherd — Lime-Silica Seines of Minerals. 273 



above that point the higher the temperature, the more rapid 

 the melting. A charge of quartz was heated for a long time 

 in a platinum furnace at 1555° without producing a trace of 

 fusion. 



There is little satisfaction in pursuing an inquiry of this 

 kind. As has been stated elsewhere with reference to an 

 entirely similar case,* the term "melting point" does not 

 appear to be well applied to cases of this character, in which 

 the crystalline structure persists for days or weeks at tempera- 

 tures above the point where melting begins. f If the change 

 of state is to be defined by the absorption of heat, and the 

 absorption of heat extends over a wide range of temperatures 

 and conditions, our forms of expression should be revised 

 somewhat to include these hitherto unrecognized cases. 



Tridymite. — The relation between tridymite and quartz 

 appears to be a simple one, although the literature of the sub- 

 ject is unsatisfactory. But few trustworthy observations have 

 been recorded and the conclusions drawn from them are vague 

 and contradictory. So far as known, quartz has never been 

 crystallized as such from mineral fusions except where cataly- 

 zers were present. Tridymite has probably been obtained by 

 several individuals through the accidental crystallization of 

 fused silica vessels,:): but no especial attention appears to have 

 been given to the circumstances in which it occurs, and its 

 identification has not always been positive. 



Like the melting temperature, the inversion of quartz to 

 tridymite and the crystallization of fused silica are very dif- 

 ficult phenomena to study, owing to the extreme inertness of 

 the material, but a number of experiments have been success- 



* Isomorphism and Thermal Properties of the Feldspars, Publication 81, 

 Carnegie Institution of Washington, p. 74 (3). 



f Doelter has recently offered a new general classification of silicates (loc. 

 cit. p. 3): " Einfacher konstituierte Silikate (Gruppe A) haben scharfen 

 Scknielzpunkt, geringere Viskositat und grosseres Kristallisationsvermogen. 

 Komplexere Silikate (Gruppe B) haben ein grosses Schmelzpunktsintervall, 

 grosse Viskositat, geringere Kristallisationsgesckwindigkeit;" 



Such information as we have been able to gather in our work with pure 

 minerals does not substantiate this generalization. "We have encountered 

 no mineral more viscous than quartz, which has the simplest of compositions. 

 Anortkite, wollastonite and diopside differ radically in simplicity of formula 

 but melt and crystallize at very nearly the same rate and with very nearly 

 the same sharpness. 



JProf. Dr. L. Holborn of the Physikalisch-Technische Reichsanstalt ; Dr. 

 M. Herschkowitsch of the firm of Carl Zeiss in Jena ; Dr. Kiich of the firm 

 of Heraeus in Hanau ; and the Rev. Theodor Wulf, S. J., formerly of Gottin- 

 gen, in the preparation and use of quartz glass vessels have noticed that the 

 glass deviferifies at high temperatures in the presence of water vapor or after 

 long usage. So far as we are aware, all of these observations have remained 

 unpublished. Hahn, in the Int. Cong. f. Angewandte Chemie, Berlin, 1903 

 (vol. 1, p. 714), notes the devitrification of a quartz glass tube at 1100°. He 

 also identified the crystal formation under the microscope as tridymite. 



