474 Transformations of Mechanical into Chemical Energy. 



position product, the particles protect one another. And even 

 when all other things are equal, the product will depend on 

 the activity and dexterity with which the grinding is done. 



But what is really important is that the amounts obtained 

 enable us in particular cases to determine the exact amount 

 of mechanical energy which has been transformed. 



In concluding this paper, two cases will be mentioned 

 which do not belong to the above category, inasmuch as the 

 reductions are exothermic. As a general rule, to cause sub- 

 stances to part with oxygen, energy must be supplied, but 

 there are exceptions. Auric oxide, for example, disengages 

 heat in parting with oxygen, and the same is the case with 

 potassium permanganate. 



Auric Oxide. — As just said, the reduction of auric oxide is 

 exothermic. Thomsen found the equations for its formation : 



2Au + 30 + 3H 2 = 2Au(OH) 3 -2x 6-6. 



The production, therefore, of a molecule of auric hydroxide 

 requires that 6' 6 C. be supplied. We should, therefore, 

 expect to find gold hydroxide more easily and more largely 

 reduced by shearing-stress than other oxides. 



Experiment confirms this expectation, and although this 

 case does not belong to the subject proper of this paper, which 

 deals with endothermic reactions, it seems sufficiently in- 

 teresting to add in this appendix. 



Gold trioxide hydrate, Au (0H) 3 , was obtained by Figuier's 

 method (a gold solution is rendered strongly alkaline with 

 potash or soda, and barium chloride is added as long as it 

 causes a precipitate. Part of the gold remains in solution. 

 The barium aurate is washed and decomposed with very dilute 

 nitric acid). The brown powder after drying was ascertained 

 to be completely soluble in warm dilute hydrochloric acid 

 (1 to 10), showing that no gold had been reduced. 



(1) Of gold oxide 155 mgs. were taken and were tritu- 

 rated 25 minutes in a porcelain mortar. The large reduction 

 was at once evident to the eye, the pestle looked as if plated 

 with bright gold. The unchanged portion of oxide was removed 

 by digesting with dilute hydrochloric acid. The metallic 

 gold was dissolved in aqua regia, reduced and weighed. 

 It amounted to ... . *0718. 



The 155 mgs. of gold oxide taken contained 123*1 of gold. 

 The reduction was, therefore, as 718 to 1231. Fifty-eight 

 per cent., or more than half the gold contained in the material 

 employed, was reduced. 



(2) 200 mgs. gold oxide were triturated 30 minutes in a 



