54 LIFE: ITS NATURE AND ORIGIN 



work, rediscovered gold sols and used them to measure the relative 

 protective power of various colloids. The "gold number" repre- 

 sents the number of milligrams of protector which just fails to pre- 

 vent the coagulative color change, from red to violet, of 10 cc of 

 an extremely pure and highly sensitive colloidal gold solution, 

 upon the addition of 1 cc of a 10 per cent solution of sodium 

 chloride. The use of these sols in the Lange test has been men- 

 tioned above. The gold numbers of a few protectors is given 

 herewith: 



The generally accepted explanation of the mechanism of prot- 

 tection is that advanced by H. Bechhold, 24 i.e. that the protector 

 is adsorbed at the surface of the protected particle. The nature of 

 the outwardly directed electronic fields is evidently an important 

 factor also, for Bechhold later found that certain minimal quanti- 

 ties of protectors sensitize rather than protect. Thus 0.0003 to 

 0.0001 part of gelatin per million will flocculate gold sols and oil 

 emulsions. This principle is used in the mining industry (flota- 

 tion processes), and also to flocculate coal slurries. 25 The work 

 of J. Billiter 26 indicates that these traces of protector may bring 

 the colloid particles to the isoelectric point, where, as Sir William 

 B. Hardy has shown they are especially susceptible to flocculation. 



Another curious quirk in the effects of adsorbed surface layers 

 becomes evident in other cases where the newly formed surfaces attract 

 each other strongly and stick together. Alexander 27 considered starches 

 as a mixture of substances: (1) amylopectin, the more coherent, less 

 dispersible gel-like portion (now known to contain branched mole- 

 cules), and (2) amylose, the more soluble or dispersible portion, now 

 known to contain unbranched molecules. 28 In discussing colloidal 

 protection as a factor in the behavior of starches, it was pointed out 

 that the behavior of a substance or a mixture of substances depends 

 largely on the nature and relative proportions of its aggregating and 

 its protective fractions. The term cohesive colloid was suggested for 



