and JRoclc Densities at High Temperatures. 19 



crystal angles (F. E. Wright, unpublished), double refraction, 

 refractive index parallel and perpendicular to the axis, and 

 specific heat. While some of these properties are interdepend- 

 ent, nevertheless it is a striking fact that in every case the 

 effect of the change at 575° begins to be felt 200° or more 

 below the temperature of complete inversion. 



All of these various properties agree in showing a sudden 

 break in the curve at 575°. In other words, although each 

 property shows a rapidly increasing effect due to the approach- 

 ing inversion, nevertheless there is apparently a discontinuity 

 in the low-temperature branch of the curve. The final stage 

 of the transition occurs very rapidly, probably within (H , a 

 range of temperature too small to be controlled and measured 

 with our present forms of apparatus. This final part of the 

 curve should therefore be drawn as a vertical straight line. 

 The optical data show this point better than do the volume 

 data. 



The form of the quartz curve is not that which would result 

 from the presence of two molecular species in equilibrium, of 

 which the one is increasing in concentration at the expense of 

 the other. It is rather the form of curve which would result 

 from the action of opposing mechanical forces. 



Various mechanical models of such a system will suggest 

 themselves. For instance, the curve showing the relation be- 

 tween the angular velocity and the moment of the forces on two 

 co axial cylinders rotating in water has a discontinuity like that 

 in the quartz curve, since there is a limiting value of the veloc- 

 ity below which the moment is proportional to the velocity, and 

 above which the square of the velocity enters. 



The quartz curve therefore tends to confirm the view, well 

 stated by Fenner in his article on the forms of silica,* that 

 the alpha-beta inversions of quartz and tridymite represent 

 only a rearrangement of the molecules in the crystal, whereas 

 the change from one of these forms to another represents a 

 real change in the constitution of the molecule itself.f 



The unusually small variation in the properties of the high 

 temperature quartz with, rising temperature is also striking. 

 It is pertinent to inquire whether there may not be some con- 

 nection between this fact and the unusually slow time-rate of 

 melting of quartz. In both of these respects it differs from 

 almost all other known solids. Orthoclase shows an interesting- 

 parallelism in being, on the one hand, one of the slowest- 

 melting of the silicates, and, on the other, in having one of 



*This Journal, xxxvi. 385, 1913. 



f Since the above was written, F. E. Wright has published a comparison 

 and discussion of the quartz curves. Jour. Washington Acad. Sci., iii, 485- 

 494, 1913. 



