202 Dr. Playfair on Transformations 



expected the oxides of nickel and cobalt to have exerted the 

 same power, but from Thenard's description of the former 

 being in the state of a black powder, it may have been the 

 oxide of increased specific gravity, to which attention has 

 already been drawn*. In all these cases the affinity is sup- 

 posed to be sufficiently strong to break up the atoms of a 

 body yielding to the slightest disturbance of its state of stati- 

 cal equilibrium. Two affinities are at play in these decompo- 

 sitions, viz. the attraction of the metaUic oxide for oxygen 

 and that of the water for the same body ; both these affini- 

 ties resist the union, and therefore, elasticity coming into 

 operation, robs both oxides of the gas. The affinity causing 

 the decomposition is so slightly preponderating in its in- 

 fluence, that a second cause coming into operation is quite 

 sufficient to alter the conditions under which it was originally 

 exerted, and to draw one of the elements of the body acted 

 upon beyond the sphere of its affinity. 



The balance of affinities in all such cases is so near that we 

 not unfrequently find apparently contradictory effects result- 

 ing from their gratification. Thus the addition of oxide of 

 silver to peroxide of hydrogen expels oxygen from the latter, 

 but at the same time it is robbed of its own oxygen and re- 

 duced to the metallic state. In this case we have two feeble 

 compounds instead of one, with affinities very nearly balanced, 

 and with atoms so tense as to yield readily to the first dis- 

 turbing cause. We can scai'cely adopt as sufficient the ex- 

 planation of Thenard and Mitscherlichtj that the reduction 

 is due to the elevation of temperature accompanying the de- 

 composition, because even when that is lowered by the ad- 

 dition of much water to the peroxide of hydrogen, the silver 

 still becomes metallic. 



It is a point yet undetermined, whether a lower oxide is to 

 be considered as unity to a higher oxide, or whether all the 

 atoms of oxygen are held by equal attractions. We know 

 that tartaric acid is able to separate potash from nitric acid 

 in forming a bitartrate, and yet acetic acid is sufficient to re- 

 move the second atom of potash from the neutral tartrate. 

 But in a bibasic acid, like tartaric acid, it may be either atom 

 of potash that is abstracted, and the superior affinity for the 

 remaining one may be owing to attractions resulting after 

 the expulsion of the first. Thus MnOg may have its atoms 

 of oxygen distributed round the central nucleus Mn, and held 

 by equal attractions, and the stability of the red oxide pro- 

 duced by its calcination does not show that it pre-existed 



* Memoirs of the Chemical Society, vol. ii. p. 381, and vol. iii. p. 81. 

 f Poggendoi-fF's ^nwa/e«, Iv. 321. 



