RECENT ADVANCES IN SCIENCE 375 



vestigated by W. B. Hardy {Phil. Mag., 1920, [vi], 40, 201). 

 The static friction between a burnished bismuth surface and a 

 bismuth slider, lubricated by a large number of pure sub- 

 stances, has been determined. It is shown that, although the 

 static friction is a function of the molecular weight of the 

 lubricant, an important part is played by chemical constitution. 

 In some simple chemical series, the static friction diminishes 

 regularly as the molecular weight increases, and in the case of 

 chain compounds such as fatty acids, alcohols, and paraffins, 

 it appears that a good lubricant will be found if one goes high 

 enough in the series. The presence of the dOYi and COOH 

 groups, however, disturbs this simple linear relation to the 

 molecular weight. The first members of a homologous series 

 are frequently abnormal, and in the case of the fatty acids an 

 alternation in the value of the static friction occurs which is 

 similar to that observed in the melting-points of this series. 

 The presence of unsaturated groups increases the lubricating 

 power of chain compounds, whether it be the double bonded 

 oxygen of ketones or acids or the carbon of olefines and 

 alcohols. 



In their qualities as lubricants of bismuth, ring compounds 

 are the converse of chain compounds ; thus the presence of 

 double bonds and OH and COOH groups decreases the lubri- 

 cating power. It is observed that no ring compound which 

 was investigated is a good lubricant ; a much smaller change 

 in the static friction is obtained with increase in molecular 

 weight, and even cholesterol, with a molecular weight of 366, 

 is a poor lubricant. It is also shown that the more efficient 

 the lubricating power of a substance, the more strongly is it 

 absorbed by a bismuth surface. 



Colloidal Electrolytes. — McBain has recently summarised 

 his work on the constitution of soap solutions (McBain and 

 Salmon, J.A.C.S., 1920, 42, 426). From this investigation it 

 appears the solute of a soap solution contains at least five 

 different constituents ; the crystalloid soap and its ions, the 

 colloidal micelle with a large negative charge, and the typical 

 colloidal particle, which is neutral or only slightly charged. 

 Of these the greatest amount of interest is attached to the 

 colloidal micelle. In colloidal electrolytes the micelle plays the 

 same part as that of the simple ion in crystalloid electrolytes ; 

 it is a heavily hydrated polyvalent particle that carries a large 

 number of electrical charges and conducts electricity just as 

 well or even better than the simple ion it replaces. This high 

 mobility of the colloidal ion explains the high conductivity of 

 concentrated soap solutions. McBain regards the micelle as 

 an agglomeration of the fatty ions of the soap, which, owing to 

 the enhanced electrostatic potential, is heavily weighted with 



