452 SOLUTION AND THE CHEMICAL PROCESS. [XVIII. 



union, giving rise to new species, we have double decomposi- 

 tion. In the case of chloride of arsenic, the aqueous solution 

 exhibits the first stage of the process. A similar condition of 

 unstable union is observed in many other instances; thus 

 binoxide of manganese gives, with cold hydrochloric acid, a 

 brown solution, but the combination is by a gentle heat re- 

 solved into chlorine gas, and a rose-red solution of protochloride 

 of manganese. So a mixture of equivalent parts of chloride 

 of benzoyl and benzoate of soda combines at a temperature of 

 130 C., to form a limpid solution, and it is only on raising 

 the temperature that the precipitation of sea-salt indicates the 

 commencement of that decomposition which yields at the same 

 time. anhydrous benzoic acid.* It is only when looked upon 

 as a momentary combination followed by a decomposition, that 

 the theory of double decomposition becomes intelligible, and is 

 in accordance with known facts. 



From the narrow limits of temperature which often include 

 the two processes, and from the ease with which light, warmth, 

 friction, and pressure excite the decomposition of such bodies 

 as the chloride of nitrogen, the nitrite of ammonia, the oxides 

 of chlorine, and the metallic fulminates, we may conceive that 

 within still narrower limits, and under conditions as yet unde- 

 fined, many bodies may exhibit affinities for each other which 

 are reversed by a very slight change of condition. In this 

 way we may explain many of those obscure phenomena hith- 

 erto ascribed to action by presence or catalysis. 



* Gerhardt, Ann. de Chimie et de Physique, 3 m Serie, Tom. XXXVII. 

 page 299. 



