390 Scientific Proceedings, Royal Dublin Society. 
density 2:2, without exhibiting any observed phenomena indica- 
ting a state of strain, seems to suggest that such silica comes very 
near to the above-mentioned limit, where the curve indicating 
volume change flattens out (fig. 4). But I have to return to 
this matter again later. Only in the meantime I wish to point 
out that we cannot safely infer that the melting phenomena of 
quartz are the same as those of amorphous silica, although the 
actual melting point, as defined above, of the two substances may 
be nearly identical.1_ It would be interesting to know if any 
difference in the rate of melting or period of softening of quartz 
and amorphous silica could be detected on the meldometer. 
I therefore contend that quartz must exhibit a sudden volume 
change on melting which is, in all probability, accompanied by a 
sudden absorption of heat, and a sudden change of other physical 
properties (fig. 2, and Plate XXII.). M. Pionchon’ measured the 
heat absorption of quartz up to 1185° C., and found an increase of 
specific heat in the neighbourhood of 400°, but from that up to 
nearly 1200° the specific heat was constant. This, I believe, 
indicates that the melting point of quartz is certainly above 
1200°. 
3. Professor Joly introduces viscosity (as indicated by ductility) 
as a criterion of melting. Dr. C. Barus* again deserves to be 
heard on the subject of viscosity in liquids and solids:—“ In a 
liquid or a viscous fluid under moderate stress the instabilities are 
supplied by the mere thermal agitation at ordinary temperatures, 
at the same rate in which they are used in promoting viscous 
motion. Hence viscosity is constant at a given temperature. In 
a solid under stress the instabilities are expended at a rate 
decidedly greater than the small rate of continuous supply. Thus 
viscosity decidedly increases with time.” Or, again, in referring * 
1Cf. R. Cusack, ‘‘On the Melting Points of Minerals,’’ Proc. Roy. Irish Acad., 
Ser. ur., vol. iv. (1896-98), p. 407. It is quite certain that the melting points of 
polymorphic bodies are not in general identical, although it is conceivable that in certain 
cases they may be. Cf. Van ’t Hoff, Lectures on Theoretical and Physical Chemistry, 
pt. ii., p. 134. London (1899). 
2M. Pionchon, ‘‘ Sur la variation de la chaleur spécifique du quartz avec la tem- 
pérature.’? Comptes Rendus, 106 (1888), p. 1344. 
3 C. Barus, Phil. Mag. [v.], xxix. (1890), p. 354. 
4 C. Barus, ‘‘ Isothermals, Isopiestics, and Isometrics relative to viscosity,’’? Amer.. 
Journ. Sc., 1.° 45 (1898), p. 88. 
