204 CHEMICAL DISCOVERY AND INVENTION 



addition of hydrogen into methane or marsh-gas, CH 4 . Aniline 

 is also converted by similar action into hydrocarbons and 

 ammonia which is, of course, produced from the nitrogen de- 

 tached. 



Professor Sabatier explains the action of these metals by the 

 hypothesis of the formation of an unstable temporary hydride 

 of the metal formed by combination of hydrogen with the super- 

 ficial layers. Such hydride would be easily dissociable, and the 

 hydrogen, therefore, is easily removed by contact with un- 

 saturated compounds. 



This hypothesis is in harmony with the views which seem to 

 prevail about catalytic processes in general. In all cases which 

 have been sufficiently investigated there appears to be formed a 

 small, often minute quantity, of an unstable and often merely 

 temporary compound which seems to carry its effect from 

 molecule to molecule throughout the mass, sometimes remaining 

 recognisable at the end or merely reverting to the condition of 

 the original catalytic agent introduced. It is, however, often 

 difficult to determine in special cases whether the action is 

 attributable to the formation of a definite though unstable 

 chemical compound, or whether it is to be included among those 

 still obscure cases of physical condensation which come under 

 the modern designation " adsorption." 



It has long been known that metallic palladium can " occlude," 

 to use the common expression, several hundred times its volume 

 of hydrogen gas. The metal retains its ordinary appearance after 

 being charged either by acting as the cathode in an electrolytic 

 cell decomposing acidified water, or by heating the metal in 

 hydrogen gas. This palladium-hydrogen immersed in a solution 

 of ferric chloride reduces it to the state of ferrous chloride, again 

 without visible change in the palladium ; is this to be regarded 

 as a compound of palladium and hydrogen in which the hydrogen 

 can become active in consequence of being ionisable ? 



Hydrogen gas, under ordinary circumstances, is without 

 action on a ferric salt, but under considerable pressure it pro- 

 duces reducing effects. If materials capable of generating 

 hydrogen, such as zinc and dilute sulphuric acid, are brought 

 into contact with such a compound as ferric chloride, reduction 

 occurs, and the result is commonly spoken of as an effect of 

 " nascent " hydrogen, which is then supposed to be in the state 

 of free atoms electrically charged. There is apparently a little 



