564 Dr. Armstrong and Prof. Armstrong. Studies on [June 13, 



which produces chemical change in another or other substances merely by- 

 contact, though it in no way has this significance etymologically. This idea 

 has gradually supplanted that of a loosening down preparatory to and 

 determining chemical change between substances in contact with the 

 catalytic agent — the conception which appears to have been in the mind of 

 Berzelius when he coined the term from the Greek Kara and \vo. The 

 interaction of hydrogen and oxygen or of sulphur dioxide and oxygen in 

 presence of platinum are cases in point : the " loosening down " of the 

 molecules concerned is commonly overlooked and the combination effect 

 alone thought of. 



The definition which has been popular of late years is that quoted above 

 from Euler but originally advanced by Ostwald. No proof whatever is 

 forthcoming, however, that the catalyst merely alters and hastens the rate of 

 a change already in progress. Whilst such an assumption is permissible, it is 

 in no way necessary. A very large amount of evidence is on record showing 

 that, in many cases in which it has been customary to speak of two substances 

 as interacting, the change only takes place under special conditions in 

 presence of a third substance of a particular type — in other words, that the 

 catalyst determines the change : Brereton Baker's work, in particular, has 

 afforded much proof of this kind but such evidence is entirely disregarded by 

 the advocates of the view that the catalyst merely hastens change and it has 

 nowhere received proper notice. 



When chemical changes are regarded as electrolytic phenomena, as the 

 several components of the system are all concerned in the change, it is 

 obviously not an easy matter to decide which of the substances present is to 

 be regarded as the catalyst. The interaction of hydrogen and oxygen, for 

 example, is not determined by platinum alone but only takes place when an 

 electrolyte is present to complete the circuit : the " loosening down " which 

 Berzelius appears to have contemplated takes place immediately the circuit is 

 formed and probably is consequent on the various attractions reciprocally 

 exercised throughout the circuit. The platinum undoubtedly acts either by 

 absorbing them at its surface or by combining with each of the two gases, 

 thereby bringing them into circuit with the electrolyte. 



On this account, the catalyst may well be defined as the agent which brings 

 about the inclusion of the interacting substances in the circuit within which 

 the change takes place so soon as the circuit is established, the electrolyte 

 being the actual agent by which the change is effected. 



Obviously, however, the electrolyte may, in some measure, be regarded as 

 the catalyst and, as a matter of fact, it is generally so regarded in the case of 

 the hydrolysis of ethereal compounds by acids. 



