CATALYSIS AND BIOCHEMICAL PROCESSES 231 



from n-propyl up, esters, urethanes, hydrocarbons, 

 chloroform, nitromethane, ethyl ether, chloretone, phenyl 

 urea, and others.^ 



In general these effects in lecithin suspensions are 

 referable to several factors, of which the chief probably 

 are solution of the compound in the colloidal particles, 

 formation of adsorption-films which change the electrical 

 polarization or other physical constants of the particles, 

 and increase of viscosity. The total effect in most cases 

 is increase in the physical stabihty exhibited by the 

 system in the presence of conditions tending to alter 

 the state of aggregation. 



The protective influence exerted by proteins and other 

 surface-active colloids in the precipitation of metallic 

 hydrosols by electrolytes is apparently a closely related 

 phenomenon. Its conditions have been studied care- 

 fully by Zsigmondy,^ who finds wide variations in the 

 effectiveness of different compounds; e.g.., gelatine is 

 highly effective as compared with peptone. He assigns 

 to each protective colloid a characteristic ^'gold number" : 

 this number defines the quantity of the colloid required 

 to prevent the precipitation of a standard gold suspension 

 by a definite concentration of NaCl. In this case the 

 protective effect undoubtedly depends on the formation 

 of adsorption-films, which prevent coalescence or flocking 

 of particles. Adsorption-films of soap, protein, or 

 similar substances play an analogous part in the forma- 

 tion of emulsions, as already pointed out, the suspended 

 droplets being thus prevented from fusing. In a similar 

 manner these substances prevent sedimentation in finely 



^ Cf. my The Theory of Ancesthesia, p. 361. 

 * Zsigmondy, Colloids and UUramicroscopy. 



