112 LIFE: ITS NATURE AND ORIGIN 



Increase in thermal agitation increases the total number of en- 

 counters per unit of time between reactant catalyst and substrate 

 areas, and this works in favor of increasing the number of fruitful 

 encounters. As the temperature increases, however, a point is 

 reached where so many particles have so high a relative velocity 

 that the number of unfruitful encounters increases to such an 

 extent that the catalyst efficiency per unit of time tends to fall off. 



Many other intercurrent factors may influence the thermal optimum 

 in catalysis besides the kinetic factors; e.g., the catalyst, reactants, or 

 end products may undergo decomposition; adsorbed substances may 

 "lame" the catalyst; specific substances or ions may affect the degree 

 of dispersion or electronic integrity of catalyst or reactants. Thus 

 Svedberg found that many proteins are dispersed into smaller sub- 

 units by change in pH, some of them being reconstituted from the 

 fragments when the pH is shifted to the stability zone; and many sub- 

 stances will "dissolve" or disperse glue at room temperature, e.g., 

 calcium chloride, sodium nitrate, sodium naphthalene sulfonate. 



Pepsin is "activated" by hydrochloric acid. What this means ki- 

 netically may in part be followed ultramicroscopically. A dilute solu- 

 tion of egg white heated nearly to boiling gave an opalescent dis- 

 persion full of bright, rapidly moving ultra-microns. On allowing a 

 droplet of pepsin solution (Fairchild's containing 15 per cent alcohol) 

 to diffuse under the cover glass of the slide, the albumin ultramicrons 

 immediately coagulated into large, motionless masses. When a drop- 

 let of O.IN hydrochloric acid was introduced under the cover glass 

 the coagulated masses burst into small groups and isolated ultra- 

 microns, which resumed their active kinetic dance. But almost im- 

 mediately the albumin particles began to grow fainter and to dis- 

 appear, the field meanwhile becoming brighter as smaller ultramicrons 

 or amicrons were formed. The addition of pepsin to the opalescent 

 albumin solution caused it to clear gradually at room temperature. 



The kinetic motion of diastase particles may likewise be followed 

 ultramicroscopically, as they gather about and actively rub or "gnaw" 

 holes or cavities in starch granules. The chemical changes occurring 

 are, of course, far below the level of visibility. 



Haurowitz and Schwerin 27 found that the catalytic auto-oxidation 

 of linoleic acid by hemin in heterogeneous emulsions ceases when the 

 system is made homogeneous by addition of acetic acid, dioxane, 

 pyridine, alkali or bile, but recommences when homogeneity is 

 abolished by addition of water or stronger acid. These authors 

 believe that this reversal demonstrates that the catalysis takes place 

 only in the interfacial film between the water and oil phases, and 

 attribute the observed diminution of the velocity of oxidation by 



