CATALYSTS 93 



size, the same mass of metal may be packed into the box regardless 

 of the size of the spheres, provided they are uniform in size. Mass 

 and total effective volume are not altered, but surface is increased. 

 The surface of a sphere is 4>7rr 2 . If the original sphere be divided 

 into 100 small shot, then the new surface would be 100 X4?rr 1 2 

 where r^radius of small shot. Now r 1 ^= v / T ^ 1J =4-64, i.e. the 

 surface would be increased over four and a half times. If the 

 subdivision were carried still further till there were 10 30 small 

 shot, then the total adsorbing surface would be increased 

 10,000,000,000 times. The intensity of adsorption is chiefly 

 dependent on the area of adsorbing surface (cf. Table, p. IX.). In 

 other words, contact catalysis is indicated where the specific 

 surface of the catalyst comes within the colloidal range. Charcoal 

 is used as an adsorbent in the clarification of sugar. A cubic 

 metre of charcoal consisting of particles 1 mm. in diameter has a 

 surface of about 600 sq. metres. If the particles are reduced to 

 colloidal dimensions, say to 0-1 /u. diameter, then the adsorbing 

 surface becomes 60,000,000 square metres. 



2. Carriers. In some cases the catalytic agent combines 

 chemically with one of the reacting substances to form an un- 

 stable intermediate compound. This, in turn, breaks up, regener- 

 ates the catalyst, and liberates the reagent in the active atomic 

 state so called nascent. Many oxidations and reductions are 

 brought about in this way. 



3. Ionic Catalysts. Hydrogen and hydroxyl ions act as cata- 

 lysts for many reactions which occur in aqueous solution. The 

 velocity of such a reaction in dilute solution is proportional to 

 the concentration of the ions in question, provided the thermo- 

 dynamic environment remains constant. The ion probably 

 acts as a carrier, forming an unstable perhydrate as intermediate 

 product. 



The great majority of vital catalytic reactions have, as catalyst, 

 an enzyme. Enzymes themselves cannot be detected or estimated. 

 Their presence is made apparent by their action. By estimating 

 the amount of the products of enzyme activity an idea of the rate 

 of reaction may be gained. Many attempts have been made to 

 isolate and purify certain enzymes and, though complete success 

 has not been granted to any investigator, much has been learned 

 of their nature and of the conditions necessary for enzyme action. 



(a) Enzymes are colloidal. They can readily be separated from 

 crystalloids by dialysis or ultrafiltration. Chemically, they 

 resemble their substrate or are so closely associated with their 



