Thermal Hysteresis of Fused Silica. 725 



all temperatures above about 1000°, quartz and fused silica 

 devitrify into crystalline tridymite*, which above this 

 temperature is the stable phase. 



At —80° Scheel noted a maximum density as well as a 

 minimum length. There is no analogue of this point in the 

 expansion curve of crystalline quartz f . 



As to the existence of a third change-point in fused silica 

 at about 500°, the expansion curve, it is true, indicates a 

 maximum value of the coefficient of expansion at that tempe- 

 rature. But the maximum is not very pronounced and the 

 certain existence of a change-point can scarcely be inferred. 

 It is useful to note Mallard and Le Chatelier^s J measurements 

 on the expansion of quartz crystal at high temperatures. 

 Their results when plotted give an expansion curve which 

 steepens up rapidly § in the neighbourhood of 500° to a well 

 marked maximum length at about 670°; in fact, the expansion 

 curve of crystalline quartz at 500° is very much like that of 

 fused quartz at 1100°. 



A recalescence point at about 500° is well marked with 

 quartz crystal, but a thermo-junction embedded in powdered 

 fused silica does not, according to Rosenhain, support the 

 same claim for fused quartz. 



Perhaps it would be right to infer that the maximum at 

 500° in the expansion curve of fused silica is a residual 

 effect, and that the change from crystalline to amorphous 

 quartz, though apparently complete as judged by other 

 tests ||, is not so complete as to avoid recognition by the very 

 delicate means that Fizeau's method affords. 



However, the point, such as it is, may be useful in setting 

 a limit to the temperature that should be employed in anneal- 

 ing a silica standard of length intended for use at ordinary 

 temperatures. Moreover, as will be seen later, the thermal 

 hysteresis exhibited by silica is much less for temperatures 

 below about 500° than for higher temperatures. Apropos of 

 this, Blackie's observation (above) as to the reversal at 500° 

 of the relative expansibilities of the transparent and trans- 

 lucent varieties of silica may also be noted. 



* The densities of the various varieties of silica are : — Quartz 2*65, 

 tridymite 2-32, transparent fused silica 2*21, translucent fused silica 2-07. 

 The optical constants and crystalline forms of quartz and tridymite are 

 very similar. See Dana's ' System of Mineralogy.' 



t See Scheel, Deut. Phys. Gesell. Verh. ix. p. 3, Jan. 1907. 



X Mallard and Le Chatelier, Compt. Rend, cviii. p. 104(3 (1889). 



§ See also Randall, Phys. Rev. xx. p. 10 (1905). 



|| Mr. Blackie has obtained some interesting results at the N. P. L. 

 from a microscopic examination with polarized light. 



