392 Scientific Proceedings, Royal Dublin Society. 
Biot,! the latter of whom refers to the work of Brewster and Sir 
John Herschel. The conclusion arrived at was that this rotatory 
power was a property characteristic of silica crystallized as quartz. 
I therefore submit that quartz is certainly not decrystallized, even 
in the neighbourhood of 1500°. It cannot, then, I think, be re- 
garded as a crystallizer at low temperature. In fact, I see no 
reason for lowering the melting point of quartz as given by the 
meldometer, @.e., 1425°. 
It may here be well briefly to consider the melting points of the 
mineral silicates.” In the absence of more complete data one would 
hesitate to go beyond the indications of the meldometer, as given 
by Dr. Joly* and Mr. Ralph Cusack. They certainly remain the 
best measurements that we have got. But in the paper under 
discussion, Dr. Joly seems almost inclined to rob them of all value 
by suggesting that the ordinary gravitational and surface tension 
forces may not be sufficient to make melting apparent. But 
how are we to know when they are sufficient and when not? 
Of what use, in fact, is the meldometer ? 
Mendeléeff,’ as Dr. Joly points out, has likened the silicates to 
metallic alloys, regarding them as “‘ alloyed”’ oxides—Ca0O, A1,0,, 
Si0,, ete. Now, so far as I know, it is characteristic of the 
majority of metallic alloys to have a melting point decidedly below 
the mean of the melting points of the constituent metals. The 
curves’ illustrated on pp. 393 and 396 (figs. 5 and 6) exhibit this 
i a striking manner. They are a few convenient examples, taken 
from Landolt and Bornstein. In many cases the melting point 
is far below that of either constituent. Two curves (Pt and Au, 
'M. Biot, ‘Sur la cause physique qui produit le pouvoir rotatoire dans le quartz 
cristallisé.”” Comptes Rendus, 8 (1839), p. 683. 
* Vide Nature, vol. Ixii., p. 368 (16th August, 1900). 
3 J. Joly, loc. cit. 
4R. Cusack, Joc. cit. 
° D. Mendeléeff, ‘‘ Principles of Chemistry.’’ Translated by Kamensky and Lawson, 
vol. 1i., p. 117. New York, 1897. 
® Plotted from data collected in Landolt and Bornstein’s Physikalisch-chemische 
Tabellen, pp. 159-161, and copied into Gray’s Fhyeical Constants (Smithsonian 
Institution, Contributions to Knowledge). 
