produced by Catalytic Bodies. 205 



nitric acid. The tin eliminating some nitric oxide quickens 

 the action, which commences with difficulty with pure nitric 

 acid ; nitric oxide gas passed through the solution answers 

 the same purpose. 



This accessory affinity also enables oxide of copper or per- 

 oxide of manganese to evolve copious streams of oxygen from 

 chlorate of potash in a state of fusion. The heat of fusion 

 decomposes the compound slowly, but on adding a body ha- 

 ving an affinity for the element acted upon by the heat (oxy- 

 gen), the decomposition proceeds with greatly increased ra- 

 pidity. We cannot ascribe this action to the presentation of 

 points from which the gas may escape, as in the lowering of 

 the temperature of ebullition by particles of sand, because 

 silica has no influence in accelerating this decomposition*. 



In the examples previously given we have the decomposi- 

 tions aided by the tendency of one of the bodies to assume 

 the elastic form. But when the body acted upon has two 

 elements, one of which is influenced by elasticity, the other 

 by cohesion, we find it peculiarly liable to be acted upon by 

 external agents. Persulphuret of hydrogen is a compound of 

 this class, and has been closely studied in its decompositions 

 by Thenardf. The same bodies which decompose peroxide 

 of hydrogen act catalytically upon this sulphuret. The de- 

 composition cannot be due to points for the escape of gas, as 

 suggested by LiebigJ, to explain the decomposition of per- 

 oxide of hydrogen, because solutions of the alkalies act with 

 equal power. The sulphurets, especially those of the alkaline 

 metals, decompose it very readily. As in the case of per- 

 oxide of hydrogen, the acids afford stability to its sulphur 

 analogue. In the view of acids given, they are supposed to 

 have become chlorous or electro-negative, representing and 

 behaving as oxygen, and therefore exerting no affinity, we 

 should anticipate that they would not show any disposition 

 to break up an oxygenous compound or its analogue of sul- 

 phur. Another instance of accessory affinity is seen in the 

 nitrosulphates § ; the formula (RO, SO^ + NOg) given by Pe- 

 louze to these compounds does not allow us to understand 

 their decompositions, which however becomes intelligible if 

 we view nitrosulphuric acid as nitric acid, in which the fifth 

 atom of oxygen has been replaced by one of sulphur (RO, 

 NO4 S). In this acid we have two elements — the nitrogen 

 and the sulphur — sharing the oxygen, their mutual affinities 

 being nearly balanced when the acid is united with an al- 



* Taylor's Scientific Memoirs, vol. iv. p. 9. 



t Ann. de Ch. et de Ph. xlviii. 79. + Ann. der Pharm, ii. 22. 



4 Ann, de Ch. et de Ph. Ix. 151. 



