404 Scientific Proceedings, Royal Dublin Society. 
Precisely the same ground has been covered in greater detail 
by Mr. C. E. Stromeyer,! in a paper read before the Manchester 
Literary and Philosophical Society in January last (referred to 
by Dr. Joly on March 21st). I need not recapitulate all his argu- 
ments which appear on the whole to be sound, though occasionally 
somewhat loosely stated. The well-known formula connecting 
the rate of change of melting point and pressure’ is— 
dO O(% — %) 
dp TOs 
where = rate of rise of melting point with pressure ; 
@ = the melting point on the absolute scale of temperature; 
(v — %) = the volume change on melting ; 
es 
i 
the latent heat of melting (in mechanical units). 
In order to apply this to the particular cases of the rock-forming 
minerals, we want certain experimental data. Dr. Joly and Mr. 
Ralph Cusack have supplied us with the melting points of the most 
important minerals (@). But the all-important items are the 
ie 
L 
Mr. Stromeyer states (1. c. p. 4), simply the volume changes; for, 
although the latent heats of the minerals may be small (1. c. p. 10), 
yet their relative magnitudes are quite as important as the volume 
changes). Now, these two quantities (volume change and latent 
heat of melting) are extremely difficult to measure directly in the 
case of bodies melting at such high temperatures as do the mineral 
silicates. 
The most important part of the volume change on melting is 
different values of the ratio ” for these minerals (not, as 
instrumental in producing the peculiar characters of the natural rocks.’’? Evidently any 
purely chemical theory is quite incapable of explaining this phenomenon. ‘The 
influence of pressure at once suggests itself. 
Cf. also H. C. Sorby, ‘‘On the Direct Correlation of Mechanical and Chemical 
Forces,’’ Proc. Roy. Soc., xii. (1862-63), pp. 5388-550. 
1C. E. Stromeyer, ‘‘The Formation of Minerals in Granite,’? Mem. Manchester 
Lit. and Phil. Soc., vol. 44, pt. iii. (1900), No. vit. 
2M. A. Battelli tested the applicability of this formula to the melting points of a 
number of organic substances, some of which melted gradually. (‘‘ Influence de la 
Pression sur la Température de Fusion,’’ Journal de Phys., tom. vi. (1887), p. 90). 
The agreement between the calculated and the found is very approximate. 
