The Origin of the Theory of Solutions. 475 



porcelain mortar. After removing the unchanged oxide as 

 above, the metallic gold was dissolved, filtered, and reduced. 



Obtained '0538. 



Potassium Permanganate. — Two molecules of permanganate 

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

 the same time 28*4 cal. The reaction is therefore exo- 

 thermic. 



By active trituration a portion of the permanganate taken 

 easily undergoes reduction. Exhausted with water, a brownish- 

 black insoluble residue remains, which dissolves with effer- 

 vescence 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. 



The second was made on a like quantity of permanganate, 

 also in a porcelain mortar. The portion rendered insoluble 

 was dissolved in hydrochloric acid, and estimated as Mn 3 A . 



Quantity obtained . . '0136, 



showing that 28*25 mgs. of permanganate had been reduced 

 to Mn0 2 . 



The same quantity of permanganate treated in the same 

 way in an agate mortar gave only '0030 Mn0 2 , owing to the 

 less efficiency of the mortar. 



The reduction in the case of these last two substances are 

 exothermic ; they, however, do not take place spontaneously, 

 but require the aid of an exterior force — this aid is supplied 

 by shearing-stress. 



In these three papers a large number of reactions have 

 been described in which mechanical energy has been trans- 

 formed into chemism. The number might be extended, but 

 in practice is necessarily limited to those cases in which a 

 perfect separation can be made between the original substance 

 and the altered product. 



XLVII. The Origin of the Theory of Solutions. 

 By J. H. van't Hoff*. 



I. Historical Introduction. 



WHEN I was requested to deliver an address before the 

 German Chemical Society summarizing the work 

 which I had done in the realm of physical chemistry, it 



* Translated from the Berichte d. deutsch. chem. Gesellschaft, vol. xxvii. 

 pp. 6-19 (1894), by Dr. John Shields. 



2K2 



