398 Seientific Proceedings, Royal Dublin Society. 
attained in the flame of detonating gas (Cremer, 1892).” Prof. 
C. V. Boys! says that silica retains its form at a temperature at 
which platinum is as liquid as water. 
Admitting that the true melting and crystallizing point of 
quartz is above that of felspar, it might he urged (as is indeed 
suggested in Dr. Joly’s paper) that the “softening point” of 
quartz is below that of felspar (to confine our attention to one 
particular crystallizing pair), and that although the quartz had 
actually crystallized before the felspar, yet when the latter erystal- 
lized it became rapidly more rigid than the former, and so was able 
to squeeze it out of shape, or was in some way able to perfect its 
own crystalline form at the expense of that of the quartz. But if 
this were the true explanation of what happens when quartz and 
felspar crystallize together, how is it that quartz is sometimes idio- 
morphic and the felspar moulded on it ? 
In the first place this fact (?) wants demonstrating by experi- 
ments on both quartz and felspar. (By these names I mean the 
crystalline compounds. The use of the name “quartz” for all 
forms of silica can only lead to confusion, and is therefore to be 
deprecated.) Nor is it sufficient to find the lowest temperature at 
which any particular substance yields to a definite distorting force 
under ordinary or reduced pressure. ‘The rigidity and certainly 
the viscosity is pretty sure to vary with pressure as well as with 
temperature. Barus’ has put this to the test of experiment in the 
case of marine glue, and exhibits the results as curves. The iso- 
thermals show how markedly viscosity increases with increase of 
pressure at constant temperature. ‘The isopiestics show how the 
viscosity decreases with rise of temperature at constant pressure. 
But it is to the isometrics (lines of constant viscosity) that 
I wish to draw particular attention. They areseen to rise rapidly 
at first, and are then convex to the axis of temperature (tem- 
perature and pressure increase together). They are all similar for 
the same substance, only displaced parallel to the axis of tempera- 
ture. If now we devise an experiment with a definite distorting force 
and gradually raise the temperature until we observe a particular 
1¢. V. Boys, “‘Quartz Fibres”? (Lecture to Brit. Assoc., 1890), reported in 
Nature, xiir., p. 604 (Oct. 16, 1890). 
2C. Barus, ‘‘Isothermals, Isopiestics, and Isometrics relative to Viscosity,”’ 
Amer. Jour. Sc., 111.° 45 (1893), p. 87. 
