380 Scientific Proceedings, Royal Dublin Society. 
and latent heat, phenomena extending over a wide range of tempera- 
ture, are a further reason for caution in this connexion. Again, 
the latent heat—as a quantity of heat involved in the progress of 
certain physical changes—is in nature complicated by such thermal 
changes as accompany the attainment of the crystallized state. 
Experiments made by cooling in a calorimeter a mass of the 
melted glass do not take thisinto account. The magnitude of the: 
error involved may be small, but is unknown. 
Until something is known respecting the molecular events 
which precede the formation of the crystal, it appears to me that 
the evaluation of all these quantities and the thermo-dyna- 
mical consideration of the entire matter are hampered by grave 
difficulties. 
I may add that whether my views as to the influence of the 
silica percentage on the order of solidification of the silicates are 
generally confirmed or not, the results on quartz fibres certainly 
remove all difficulty from the occurrence of residual quartz. I 
have found more recently that quartz fibres show a steady 
viscous extensibility even to a temperature so low as about 735° C. 
Of course this does not preclude the possibility of the silica 
crystallizing at a higher temperature under conditions of repose, 
but it relieves us of any difficulty in explaining why it does not 
crystallize at 1400° C. or for that matter at considerably lower 
temperatures. 
With reference to crystallized silica—rock crystal—I may ob- 
serve that the temperatures inducing rapid melting, as given both 
by myself! and by Mr. Ralph Cusack,” are also far above the 
melting temperatures under conditions of prolonged heating, as. 
can be conclusively shown when the latter is specially looked 
for. Finely powdered rock crystal folded in strong platinum 
sheet, and exposed for twenty-four hours or even considerably 
léss in the flame of an ordinary Bunsen burner will be found 
to have adhered in a fine dust of minute slagged and rounded 
particles which cannot be wiped off. A prismatic vertical illumi- 
nator, used with a No. 7 Leitz, shows these blebs most clearly. 
1 «<The Melting Points of Minerals.’’ Proc. Royal Irish Academy, vol. ii., ser. 
iii., p. 38, 1891. 
2 [bid., vol. iv., p. 339, 1896. 
