36 A. Killen Macbeth on 



amount of the catalyst is sufficient to effect the reaction between 

 large amounts of the reagents. 



Before passing from the inclined plane analogy attention 

 may be called to a further point. We can conceive the case of 

 the plane being tilted to an angle much greater than that required 

 to set the weight in motion. But if the weight is held by a clip 

 it will not move until such an obstacle is removed. We find a 

 parallel to this in what are known as "trigger" reactions: and 

 just as no amount of oil will start the movement of the weight 

 when it is held up by an obstacle, so trigger reactions are not 

 recognised as catalytic phenomena. Examples of trigger reaction 

 are found in the super-heating of water out of contact with dust 

 and air and the crystallisation of supersaturated solutions. There 

 is, therefore, an additional criterion for catalysis that is generally 

 advocated, namely, that (3) a catalyst only affects reactions that 

 are noi'mally proceeding at a slow rate — in many cases at an 

 immeasui'ably slow rate. 



Substances which may act catalytically . In the early 

 days of the subject it was thought that catalysis was the exception 

 rather than the rule, but with increasing experiment the 

 observations quickly reversed the belief and it would now appear 

 that there are few reactions incapable of being materially assisted 

 by the introduction of a suitable catalyst. The substances which 

 act as catalysts range from the commonest to the rarest — from 

 water to osmium, platinum, etc. -Water, indeed, is a very 

 general catalyst, bringing about the interaction of many gases that 

 would not react if perfectly dry. For example, if sulphuretted 

 hydrogen (dried by its passage over calcium chloride) is passed 

 into a flask along with sulphur dioxide (dried by sulphuric acid) 

 no reaction occurs : on adding a little water to the flask the gases 

 immediately interact and sulphur is deposited. Again, carbon 

 monoxide burns to carbonic acid gas, uniting with the oxygen of 

 the air : but a jet of the dry gas will not burn in dry air. These 

 are particular examples illustrating the catalytic action of water, 



