200 Dr. Playfair on Transformations 



There cannot be any doubt that the atoms of a body may 

 be placed in a greater or less degree of tension by varying 

 conditions. The experiments of Mr. Joule* and myself on 

 AUotropism have fully proved that the space occupied by the 

 same body alters under different circumstances. It is there- 

 fore not an unreasonable assumption that the affinity of one 

 body for a particular element may be sufficiently great to 

 produce a tense state of the atoms without effecting decom- 

 position! : hence the added affinity of a second body acting 

 in the same direction may cause that change which each alone 

 could not effect. Anything that disturbs the state of statical 

 equilibrium in such a body will often effect its decomposition. 



This accessory affinity is recognised when both bodies 

 enter into union. Charcoal and chlorine decompose alumina 

 at a red heat, though neither can do so separately. In the 

 same way BoudaultJ has shown that a mixture of potash 

 or soda and red prussiate of potash oxidizes various me- 

 tallic oxides, while Mercer has for many years made use 

 of this mixture to discharge indigo-blue on calico §. Red 

 prussiate of potash (FcgCygSK) has a great disposition to 

 attach to itself another atom of potassium to become yellow 

 prussiate of potash (Fe2Cyg4K). It cannot gratify this de- 

 sire without aid ; but when assisted by a substance having 

 an affinity for the oxygen of the potash, and capable of 

 appropriating it, decomposition follows. There are often 

 cases in which the body exercising the accessory affinity may 

 be unable to effect the union, either by the influence of un- 

 favourable chemical conditions or of cohesion or elasticity. 

 Thus, in the case with which we first started, the affinity of 

 protoxide of manganese for oxygen aids in the decomposition 

 of nitrate of protoxide of manganese, and sesquioxide of man- 

 ganese remains. If the temperature during the decomposition 



* Memoirs of Chemical Society, vol. iii. p. 93. 



t The alteration in volume is best seen in those oxides which contract 

 and increase in specific gravity by tlie application of heat, for example, 

 when the brown oxide becomes the green oxide of chromium. The two 

 oxides must have a different molecular constitution, and this may be sup- 

 posed to result from the elastic powers of one of its elements and the cohe- 

 sive force of the other. The first eflTect of heat on oxide of chromium 

 must be to expand the atoms of oxygen, and removing them further from 

 the two atoms of chromium, permit the cohesive attraction of the latter to 

 be gratified. Hence the compound acquires properties dependent upon 

 cohesion, such as indiflference to union and diminished solubility. 



X Journal de Pharmacie, tome vii., 437. [Phil. Mag. Third Series, vol. 

 xxvii. p. 307.] 



§ Phil. Mag. Third Series, vol. xxxi, p. 126. In justice to Mercer, al- 

 though this does not remove Boudault's claim of prioi'ity of publication, I 

 cannot refrain from stating that the former chemist pointed out to me tlie 

 oxidizing powers of the prussiates four or five years since. 



