1855.] on Catalytic Action and Combustion. 67 



which are commonly thought to be better understood. The precise 

 meaning of the word should not be derived from its etymological 

 constitution, nor even solely from its original application, but 

 rather from the general consent of eminent chemists as exhibited in 

 their published writings. The term has been employed very 

 loosely, and some of the definitions given do not allow of reduction 

 to a common statement. Adopting the principle laid down, 

 catalysis may be defined as the action by contact of one substance 

 upon another substance, or group of substances, whereby chemical 

 changes are effected, while the first substance remains finally un- 

 changed. This will exclude from catalysis the following cases : — 



1 . The operating body does change, although it does not com- 

 bine. 



Examples. — Fermentation, as explained by Liebig. Solution of 

 an alloy of platinum and silver in nitric acid ; also of the alloy of 

 copper, zinc, and nickel, in dilute sulphuric acid. 



2. The operating body absorbs A from a combination (A + B), 

 and B cannot exist free. 



Example. — Crystallized oxalic acid is a combination of anhydrous 

 oxalic acid and water ; if cast into strong sulphuric acid, the water 

 is absorbed, and (so far as we know at present) taken into chemical 

 union with the sulphuric acid ; but the anhydrous oxalic acid 

 cannot exist free, and is resolved into equal volumes of carbonic 

 acid and carbonic oxide, which escape accordingly. 



3. The operating body takes from a substance some of its 

 elements, and combines with them after they have combined with 

 each other. 



Example. — Sulphuric acid in contact with sugar, takes the 

 elements of water from the carbon, and combines with them by 

 hydration. 



4. The operating body causes mutual changes in substances 

 A and B, whereby they become other substances, say C and D, and 

 the operating body combines with D. 



Example. — Hydrocyanic acid and water + hydrochloric acid 

 (the operating body) gives formic acid and ammonia, of which the 

 ammonia combines with the hydrochloric acid. 



C«NH + 3H0 becomes NH3+C2HO3 



and we have N H, CI + Ca H O3. 



Cases of chemical change by friction and percussion are excluded, 

 because contact is mechanically statical, whereas these are dynamical 

 in respect of the masses. A lucifer match is an example. Sir 

 James Kane, however, supposes the decomposition of iodide of 

 nitrogen by slight percussion to be catalytic ; his definition is 

 made wide enough to take it in. 



