254 



Mr. J. Joly. 



[Nov. 18, 



f Orthoclase, transparent 0*1869 



„ opaque... 0*1890 



Labradorite 0'1933 



<J Oligoclase, transparent 0'2059 



sub-translucent 0*1997 



Albite 0*1983 



Jdicrocline 0*1905 



rTourmaline, black 0*2004 



I „ brown 0*2111 



Epidote, dark-green 0*1877 



I Hypersthene, dark-brown 0*1790 



^-Amphibole, black 0*1963 



(Quartz 0*1881 



iTopaz 0*1997 



Many of these substances may, of course, be also distinguished by 

 their specific gravities. There is indeed, as might be expected, a 

 connexion apparent between the two, the quantities tending to vary 

 inversely, not only from one substance to another, but in the case of 

 variations in specimens of the same species. 



In its physical interest I think calorimetry deserves the attention of 

 mineralogists. From results which came under notice in the course 

 of the experiments on mineral crystals, I think it will be allowed that 

 questions of interest are raised. These observations are in one direc- 

 tion only, and I have not had leisure to pursue them with the thorough- 

 ness they appear to me to deserve. Many questions suggest themselves 

 on molecular freedom within the crystal which will need care and 

 preparation to answer, and I now but give in outline an account of 

 observations made, I may say, incidentally. 



The cry stall ographic interest of the thermal capacity is based upon 

 what is apparently an intimate connexion with the amount and kind 

 of freedom possessed by the atoms of the solid. This is perhaps 

 hardly unexpected. It is probable that calorific capacity, tending to 

 be reciprocal to the mass of the atom, is more concerned with the 

 intrinsic nature of the atom than with its surroundings. Still on any 

 hypothesis of a vibratory molecule it is easy to conceive that if the 

 surroundings are such as to hamper its freedom of motion the constraint 

 will be revealed in its specific heat ; that is, if we assume its increase 

 of energy to be kinetic, as in increased amplitude of vibration. It 

 might be expected then that in the crystal the atom would reveal 

 thermally greater freedom than in the amorphous state. If, in fact 

 crystalline arrangement be regarded as the result of harmony among 

 molecular vibrations, we would expect for the crystal a specific heat 

 approximating to that of the free molecule, and the more nearly 

 approaching it the more perfectly harmony prevailed in the crystal.* 

 # There are not many experimental data for comparing the specific heat of the 



