1886.] 



On the Specific Heats of Minerals. 



257 



atoms. There is more probably some difference in constraint, of a 

 structural kind, affecting the freedom of the molecules. Des Cloizeaux 

 is of opinion that the optical examination of emerald, " and above all 

 of beryl," points to a biaxial constitution.* In recently noting on 

 these same clouded green /3-beryls from our Dublin granite, I had 

 occasion to remark on their optical heterogeneousness.f The extinc- 

 tion of basal sections is not uniform between crossed Nicols ; pale grey 

 cross-hatching is apparent, and as the mineral is rotated extinguishes 

 capriciously over the field. In these patches the restoration of illumi- 

 nation is feeble and suggests imperfect coincidence of optic and 

 crystallographic axes. 



A still greater variation appears in the sp. h. of corundum from 

 the clouded to the limpid variety, basing the sp. h. of the latter on 

 Regnault's result on sapphire. It is remarkable, however, that the 

 variation is in the opposite direction — the limpid variety exhibits the 

 higher, the clouded the lower sp. h. Thus : — 



f Corundum 0'197. R. 



a < 



I Clouded crystal of corundum. . 0'198. J. 



/3 Sapphire 0217. R. 



This is a variation of 10 per cent, about. Now the analyses given in 

 Dana's ' Mineralogy' of sapphire and corundum, although variable and 

 pointing sometimes perhaps in the direction of the calorimetric result, 

 certainly warrant no such difference, while on the other hand the 

 sp. gr. of the two varieties are invariably opposed to the result being 

 lower in the common variety than in the gem. It is perhaps remark- 

 able that both these minerals, beryl and corundum, are hexagonal. A 

 third case occurs in the case of calcite. I experimented on Iceland 

 spar and on a clouded milk-white rhombohedron, and also on a 

 specimen consisting of an aggregation of dihexagonal prisms. Adding 

 the observations of others an « and ft variety of calcite is suggested. 



Sp. h. 



Sp. gr. 



Observer. 



0-2036 



2-713 



J. 



0-2044 



2-702 



J. 



0-2046 





N, 



0-2091 



2-658 



J. 



0-2086 





R. 



R f Translucent di-hexagonal prisms 

 1 Iceland spar, two specimens .... 



In addition there is an observation of Kopp's 0*206 ; it is highly 

 probable, however, from a consideration of Kopp's general results, his 

 method of working, and, indeed, his own acknowledgement, that this 

 is too low. It is likely he was dealing with /3-calcite. It is to be 

 remembered that Iceland spar is chemically a very pure form of the 

 mineral. 



* ' Mineralogie,' vol. 1, p. 366. 



f [1885] ' Boy. Dublin Soc. Proc.,' vol. 5, p. 49. 



