Mechanical into Chemical Energy. 471 



advantage that during the prolonged and forcible grinding 

 necessary, a very appreciable amount of material is removed 

 from the mortar and pestle which must be separated from the 

 product subsequently. I have therefore made some comparative 

 experiments with a large agate mortar and an agate pestle 

 provided with a stout wooden handle adapted for the use of 

 as much force as with a porcelain mortar. But even under 

 the most favourable conditions, the efficiency of such a mortar 

 is (as will be seen) only one fifth to one tenth that of a porce- 

 lain mortar of the same size. This is largely due, I think, to 

 the high polish which is very unnecessarily given to the inside 

 of agate mortars. 



It is therefore better to make use of a porcelain mortar, 

 taking adequate means afterwards to remove the material 

 abraded. 



2. The same quantity of silver oxide was triturated also for 

 20 minutes in an agate mortar. As the abrasion is inappre- 

 ciable, it was only necessary to dissolve out the unchanged 

 oxide with ammonia. There was left, metallic silver, 



•0048, 

 showing the much less efficiency of the agate mortar. 



Mercuric Oxide. — The specimen taken was examined for its 

 solubility in cold dilute (one tenth) hydrochloric acid, in which 

 it dissolved slowly but completely. 



Half a gram was taken, and after trituration the unchanged 

 oxide was dissolved out by repeated digestions with hydro- 

 chloric acid. There remained mercurous chloride, a trace of 

 metallic mercury, and, as a porcelain mortar was used, abraded 

 porcelain. The reduction-products were dissolved out by a 

 few drops of aqua regia, were filtered and precipitated by 

 hydrogen sulphide. 



Mercuric sulphide obtained . . *0354 



Corresponding to Hg .... *0305 



And to mercuric oxide .... '0329 



This, therefore (disregarding the traces of metallic mercury) , 



is the amount of mercuric oxide which underwent reduction 



to mercurous oxide. 



The oxidation of mercur} r to mercurous oxide and that of 

 mercurous to mercuric are both exothermic reactions. As 

 respects the thermic equivalent of the oxidation of mercury 

 to both its oxides, quite different numbers have been found by 

 different chemists. Nevertheless, if from these different num- 

 bers we calculate the amount of heat disengaged by the com- 

 bination of Hg. 2 with 0, we get almost exactly the same 

 figures whether w r e use Thomsen's results as modified by 

 Nernst, or those of the French chemists ; it matters little, 



