CATALYSIS: THE GUIDE OF LIFE 111 



But decrease in particle size is also accompanied by great in- 

 crease in the area of surface exposed per unit weight. The degree 

 of dispersion where this surface area exerts its most effective 

 influence before rapidly increasing kinetic activity dominates has 

 been termed the zone of maximum or optimum colloidality. 26 

 Figure 16 is a diagrammatic sketch of these relationships, based on 

 the assumption that the particles are spheres, which, of course, is 

 seldom the case. Particles have various, and even varying con- 

 tours (threads, rods, plates, aggregates); the diagram simply illus- 

 trates the principle. 



When a particle approaches an active catalyst area, two factors 

 are of outstanding importance: 



(1) the momentary electronic contours of the surfaces in apposi- 

 tion, for this determines the possibility of particulate union 

 or influence; 



(2) the kinetic velocities of translation and rotation of moveable 

 areas, which determine whether the possibility may become 

 a reality. 



The importance of this kinetic factor came to the fore in the 

 course of a lecture experiment made by Sir Ernest Rutherford 

 at the Toronto meeting of the British Association for the Advance- 

 ment of Science (1924). It was designed to illustrate how a posi- 

 tively charged atomic nucleus will repel an alpha particle emitted 

 by radium. An electromagnet, with its positive pole up, was 

 fastened to a table so that another electromagnet, swinging from 

 the ceiling with its positive pole down, could just pass over it. 

 The mutual repulsion of like magnetic poles caused the swinging 

 magnet to take a parabolic path in any off-center approach. To 

 illustrate the relatively rare recoil which occurs when an alpha 

 particle makes a direct approach to an atomic nucleus, Sir Ernest 

 took careful aim — but the swinging magnet passed completely 

 over and beyond the fixed one, probably giving a slight unseen 

 "jump" as it did so. Quickly retrieving the swinging magnet, he 

 swung it again from a lesser distance and got the expected recoil. 



From this it is evident that if a certain relative critical velocity 

 is exceeded, particles or surfaces having the power to cohere, will 

 not do so. In catalysis, a certain period of time is necessary for 

 the electronic fields of the reactants to become fitted to each other, 

 so that apart from the effects of rotation of the particles, internal 

 kinetics, and mode of presentation, the kinetic velocity of trans- 

 lation is an important factor. 



