138 



REPORT — 1878. 



of common occurrence, and of very simple compositions. The high 

 value found for white chalk, after deducting the above correction, perhaps 

 arises from a considerable amount of hygroscopic moisture contained in 

 the stone in its ordinary state ; but, if on account of its extreme porosity 



the whole (instead of half) of the correction noted in the Table is sub- 

 tracted from the specific heat there given, the result, 0*190, is less instead 

 of greater than the known value ; and no real discordance of the obser- 

 vation from the known property of this porous stone can, therefore, be 

 properly suspected, with a suitable allowance. The proportion of foreign 

 mineral (which is compact quartz) contained in the specimen of galena 

 may be calculated approximately from the specific gravity (4'90) of the 

 specimen, which is less than the usual specific gravity (7"59) of galena; 

 and the resulting specific heat of the galena alone, which appears to 

 occupy in reality scarcely a half (47 per cent.) of the volume of the 

 thick plate used in the experiments, agrees very fairly with this material 

 substitution, with the specific heat by weight of pure galena given by 

 Regnault. 



* The plates of toughened and untoughened glass were obtained from makers in 

 Berlin. Their average conductivities are respectively -00185 and -00198, showing a 

 loss of i, or of 6£ per cent, in the conductivity by toughening. The property of 

 hard-drawn metal wires is probably analogous to this, which are known to be less 

 perfect conductors of electricity than soft annealed ones. There is a sensible 

 and nearly proportional difference, also, in the specific heats by weight ; but none, 

 apparently, in the specific gravities of the two plates. (See the accompanying list, 

 of this Report.) 



