18 



showed first that potassium hydroxide has practically no effect upon 

 the surface tension against benzene. Then he found that though 

 the addition of oleic acid to benzene lowered the tension against 

 water, it had no effect on the tension against soap solution, and he 

 concluded that the surface activity of the hydrolysis alkaU was only 

 about one-fifth to one-fourth that of the undecomposed soap. 



In all these cases again there is stiU the greatest uncertainty as 

 to what it is that is adsorbed in the interface. In Spring's work, 

 referred to in the section of " Detergent Action," he concluded that 

 it was acid soap that was sorbed by lamp black, whereas filter paper 

 sorbed alkaU leaving acid soap in the solution. 



Woodmansey {Jour. Soc. Dyers Colourists, 35, 1901, 169) found 

 that the amount of base sorbed from a solution of soap was greater 

 than that of fatty acid, an acid soap remaining in the water. 



(d) The Optical Properties of Soap Solutions. 



Soap solutions constitute clear transparent solutions disturbed 

 only by the suspended insoluble particles of acid soap resulting from 

 hydrolysis, and exhibited in varying degrees by all soaps from caproate 

 upwards. Here investigation (Darke and Salmon) shows that the 

 clear liquid is not resolvable under the ultramicroscope, and that 

 the ionic micelle is invisible in the cardioid ultramicroscope. The 

 susjoended particles of course exhibit Brownian movement, the purple 

 ones being exceptionally active, the smallest green ones coming next. 

 This Brownian movement serves as an excellent index to the viscosity 

 of the liquid ; it is scarcely detectable when the soap is highly viscous, 

 and it is highly developed when most of the soap has been removed 

 from solution as by the formation of curd. It is probable that with 

 liigher resolving power innumerable white particles would be visible, 

 and that these constitute the particles of neutral coUoid. We 

 have detected them only when conditions of illumination, &c., were 

 exceptionally good, just upon the hmits of jjossible observation and 

 they exhibited a degree of Brownian movement which corresponded 

 to the fluidity of the medium. 



EarUer investigations of the optical appearance of soap solutions 

 whether microscopic or ultramicroscopic, have thus aU had reference 

 to the suspensions of insoluble acid soap produced in smaller or greater 

 amounts by hydrolysis. The frequent turbidity of soap solutions 

 has thus been an illusory proof of their colloidal nature. The 

 insoluble acid soap may sediment out almost completely upon standing, 

 leaving the bulk of the soap in prefectly clear transparent solution, 

 or it may remain partly or whoUy suspended in particles whose 

 dimensions range aU the way from coarse suspensions down to the 

 smallest resolvable in the ultramicroscope. Every change of tempera- 

 ture or of concentration or of additions which affects hydrolysis will 

 affect the amount of this product of hydrolysis. The acid soaps in 

 suspension constitute an ordinary coUoid. 



It is now clear why Mayer, Schaeffer, and Terroine observed 

 progressively more colloidal particles in any one soap by passing 

 from alkaline to acid solution. Further it becomes plain why the 

 viscosity of solutions of salts of the lower fatty acids up to the 



