250 



Mr. J. Joly. 



[Nov. 18, 



supposition that the substance is throughout the period of heating 

 coated with a film of water adiathermanous and the external surface 

 of which may be considered as appreciably that of the steam. The 

 danger then of radiation error, that is, of steam condensing elsewhere 

 than at the surface of the substance, is small. Condensation, in short, 

 may be considered as taking place by abstraction of the energy of the 

 molecule on impact with the water film. 



II. " On the Specific Heats of Minerals." By J. Joly, B.E., 

 Assistant to the Professor of Civil Engineering, Trinity 

 College, Dublin. Communicated by Professor Fitzgerald, 

 F.R.S. Received June 28, 1886. 



A number of experiments were made on minerals by the method 

 of condensation, using the form of gravimetric calorimeter described 

 in the beginning of the paper on calorimetry (p. 353). The con- 

 densation of steam being in all cases employed, the values recorded 

 are the mean specific heats between atmospheric temperatures ap- 

 proximating to 12° C, and steam temperature, about 100° C. More 

 exactly, the values recorded are the mean calorific capacities for a rise 

 of one degree between the limits ^ and t 2 , tabulated in each case. 

 The specimens dealt with were chosen as good samples of the mineral 

 free from visible impurities.* But before detailing the particulars of 

 the experiments a few notes on the discriminative value and physical 

 interest attached to this application of calorimetry may not be amiss. f 



It seems probable that the neglect of the use of the specific heat 

 constant is to be ascribed to the difficulties besetting its determina- 

 tion. Certainly if its determination was as easily effected as we 

 effect the determination of the specific gravity of a body, there are 

 on the other hand sufficient reasons to recommend its use as in 

 general of more physical value and interest than the much used 

 specific gravity. There are cases indeed where specific gravity, as it 

 is possible to obtain it, is misleading, and where specific heat gives at 

 once valuable information on the probable chemical nature of the 

 substance. Such cases would arise with bodies of loose vesicular or 

 hollow structure. No misleading variations need be introduced into 

 the thermal constant by mere conditions of volume. 



The method of condensation permits of the determination of this 

 constant with very little experimental difficulty. On the whole the 



* I have to thank Professor Sollas for the loan of useful specimens from the 

 Museum of Trinity College. 



f In November, 1883, I suggested this use of calorimetry to the Experimental 

 Science Association, Trinity College. — " On the Determination of Minerals by their 

 Specific Heats." 



