1886.] 



On the Specific Heats of Minerals. 



255 



It is here assumed that in some way similar to that in which the atom 

 passes into the compound molecule without, in general, sensible 

 change of thermal capacity, the molecule passes into the crystal with 

 only insignificant further change in thermal capacity; the crystal 

 being regarded in some sense as the' chemical combination of the 

 molecule. Thus we- expect the specific heat of the crystal to be 

 lower than that of an amorphous aggregation of its atoms under the 

 constraint of discord. This agrees with experiment, for it may be 

 stated as true generally that in the crystal the specific heat is lower 

 than in the amorphous state, and that the more perfect and complete 

 the state of crystallisation the lower the observed specific heat. 



This, the first fact brought to my notice as experiments multiplied, 

 seems to hold through the nicest indications of crystalline state as 

 facial lustre, degree of translucency, as well as through the more 

 marked indications of cleavage and form, and is- probably quite in 

 accord with the well-known fact that rolling, hammering, and such 

 operations as tend to increase the density, diminish the specific- heat 

 of metals. The same would, doubtless, be found to hold in the case 

 of the crystalline polarity conferred on metals by vibration. 



I might illustrate the fact by a considerable number of cases where 

 it happened that the specimens dealt with showed differences in 

 crystalline state. I will only quote a few, referring to the tables, for 

 other cases. It is apparent in the case of Barite : — 



Limpid crystal. 0*1092 



Sub-transparent crystal 0*1105. 



Opaque, lamellar 0*1116 



In the case of Gypsum: — 



Transparent 0*2726' 



Opaque, rough 0*2737 



Sphalerite : — 



Crystal, high lustre 0*1144 



Crystalline, divergent 0*1155 



„ massive 0*1162 



Orthoclase : — 



Limpid 0*1869 



Opaque, well-formed crystals . . 0*1886 



Massive, cleavable 0*1899 



Fluorspar, cryolite, calcite, and quartz-opal are also examples. 

 The case of opal is, however, uncertain, owing to probable presence of 



molecule in crystalline freedom with the specific heat of the molecule in gaseous 

 freedom. In the case of ice and steam, they are the same nearly ; (Regnault) ice 

 (-78° to 0?) =0 474; steam (128° to 220°) =0*480°. 



