Mechanical into Chemical Energy. 381 



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

 exothermic. Thomsen found the equation for its formation 



2Au + 30 + 3H 2 0= 2Au(OH) 8 - 2X6-6C 



The production therefore of a molecule of auric hydroxide 

 requires that 6'6C 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 interesting to 

 add in this appendix. 



Gold trioxide hydrate Au(OH) 3 was obtained by Figuier's 

 method (a gold solution is rendered strongly alkaline with pot- 

 ash 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 com- 

 pletely soluble in warm dilute hydrochloric acid (1 to 10) 

 showing that no gold had been reduced. 



(1) Of gold oxide '155 gms. were taken and were triturated 

 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, filtered, 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 em- 

 ployed was reduced. 



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

 porcelain mortar. After removing the unchanged oxide as 

 above the metallic gold was dissolved, filtered and reduced — 



Obtained -0538 



Potassium Permanganate. — Two molecules of permangan- 

 ate by reduction to 2Mn0 2 H 2 lose 3 atoms of oxygen and at 

 the same time 28'4 Cal. The reaction is therefore exothermic. 



By active trituration a portion of the permanganate taken 

 easily undergoes reduction. Exhausted with water, a brownish 

 black insoluble residue remains which dissolves with efferves- 

 cence in strong warm sulphuric acid forming a violet solution — 

 the residue is therefore manganic peroxide. 



Three estimations were made. A first rough one, after 

 trituration for 25 minutes in a porcelain mortar, gave of washed 

 and dried material which had become insoluble, 43 mgs. 



