Joty—Pressure on Separation of Silicates in Igneous Rocks. 381 
The temperature here is a bright red, and not over 1200° as 
judged by comparison with the luminosity and colour of a mel- 
dometer ribbon of platinum. But more accurate observations can 
be effected on the meldometer. I find that finely powdered rock- 
crystal spread on the meldometer ribbon, and exposed for one hour 
to a temperature of 1180° C. leaves an adherent residue of slagged 
and rounded particles, showing their slow fusion in the most 
uumistakeable manner. But even at a temperature which varied 
but little from 1100° C., an exposure of four hours produced a like 
result. 
Finally I may mention here the result of a comparison of 
olivine, hornblende, and augite with rock crystal on the meldo- 
meter under condition of prolonged heating. ‘The powders were 
all reduced to fine flour and spread thinly in adjacent patches. 
The temperature varied between the limits of 1085°C. and 1105°. 
The exposure was for 2 hours and 10 minutes. When examined by 
a Leitz No. 5, and vertical illuminator, it was found that the quartz 
alone gave conclusive evidence of slagging. Asin the previous cases 
it was visibly rounded in small particles, while no such appearance 
could be certainly detected in the case of the other bodies. There 
were some colour changes apparent. The olivine had assumed a 
pale reddish hue; the hornblende inclined to a rusty brown. The 
augite showed little change. These changes, I may observe, also 
attend the ordinary experiments, such as have been recorded by 
Mr. Cusack and myself, and which fix the temperature at which 
these bodies slag rapidly—as olivine, 1342°-1378° ; hornblende, 
1187°-1200°; augite, 1187°-1199°.1. Whether the colour changes 
observed, indicate any alteration such as would affect the 
melting point it is difficult to say. An experiment in which 
the meldometer was completely immersed in carbon dioxide and 
the temperature maintained at 1080° C. for 23 hours showed 
less coloration, and gave the same results as regards appearance of 
melting. 
Experiments on the behaviour of the previously melted silicates. 
are in hand, but these results as they stand confirm the theoretical 
views” which I have already had the honour to bring before the 
1 Cusack, Joc. cit. 
2 «« Theory of the Order of Formation of Silicates in Igneous Rocks.” Proc. Royal 
Dublin Soc. Vol. ix. (N.*.), p. 298, 1900. 
