204 Dr. Playfair on Transformations 



silver would constantly be reduced during the action of other 

 feeble acids on carbonate of silver. If, however, we suppose 

 that the pyruvic acid, Cg Hg O5, from its affinity for more 

 oxygen, exerts an attraction for that element at the moment 

 of the liberation of the carbonic acid, the decomposition 

 would be similar to those we have already considered, espe- 

 cially if the previous view of the molecular constitution of 

 salts be admitted. In that case the oxygen of the oxide being 

 attached to that of the carbonic acid, will be made highly 

 tense during the escape of the latter, and may therefore be 

 detached by a very feeble force, its elasticity finally over- 

 coming the weak affinity. An extension of the explanation 

 however strikes me as more probable, but it would be prema- 

 ture to insist upon it without being supported by experiments 

 which I have not yet been able to conclude. 



The action of metals and of charcoal on peroxide of hydro- 

 gen may be explained by the same feeble affinity. Alkalies 

 also, from their attraction for oxygen, as indicated both by 

 their capability of uniting with more oxygen and by their 

 basic power or disposition to attach themselves to a com- 

 pound behaving as an oxygenous or chlorous element, favour 

 the decomposition of HOg, while acids, on the other hand, 

 render it more stable, perhaps, as Thenard himself suspected*, 

 from there being an inferior oxide (Hg O3?). In this instance 

 the elasticity of the oxygen tends to conceal the play of affi- 

 nities by preventing combination. 



When the acting body is present in large quantity, or ex- 

 hibits an increased surface, the action goes on with propor- 

 tionate rapidity. Thus, when nitric acid is in contact with 

 starch, the action is moderate until a certain quantity of per- 

 oxide of nitrogen has been evolved by decomposition, after 

 which it proceeds with a violence difficult to control. The per- 

 oxide of nitrogen surrounding every particle of starch aids it 

 in the decomposition of the nitric acid. That this is the real 

 cause of the phaenomenon may be proved by the following 

 simple experiment. Nitric acid is heated with starch to a tem- 

 perature at which the action has a tendency to commence but 

 has not yet begun. A stream of N04or NOg is then passed 

 through the liquid, when action immediately begins with an 

 activity proportionate to the quantity of gas added. The ele- 

 vation of temperature due to the progressive action influences 

 the decomposition, by causing the atoms of nitric acid to be- 

 come more tense. Exactly the same accessory affinity is 

 used by the manufacturer of oxymuriate of tin, when he 

 adds a fragment of tin to the mixture of chloride of tin and 

 * Traite de Chemie, p. 211. 



