26 



of low-surface tension and emulsification in the case of soaps, and 

 incidentally showed that saponin emulsified through formation of solid 

 surface film instead of through low surface tension. Hillyer also 

 demonstrated the power of penetration into capillary interstices which 

 is conferred upon soap solutions by their very low surface tension. 



We have then experimental proof of the operation of two of the 

 previously assumed factors of detergent action, emulsification (parallel 

 to low surface tension), and wetting power, both ascribable to the 

 undecomposed soap itself. The third suggested factor, the action 

 of soap in making tissue and impurities less adhesive to one another 

 was also put forward again by HiUyer, but the experimental evidence is 

 entirelv due to Spring {Kolloid, Zeitsch, 1909, 4, 161; 1909, 6, 11, 

 109, 164; Arch. Sci. phys. nat., 28, 569; 29, 41, 36, 80; Rec. trav. 

 chem., 29, 1; Bull. acad. Boy. Belg., 1909, 187, 949; 1911, 24, 17). 

 Goldschmidt has already postulated a protective action of the 

 colloidal soap upon dirt particles, since Zsigmondy had shown that 

 the gold number of sodium stearate (the minimum quantity to protect 

 10 c.c. of red gold sol from colour change upon addition of 1 c.c. of 

 10 per cent, sodium chloride solution) was 10 mg. at 60° and 0-01 mg. 

 at 100°. The value for sodium oleate was 0-6 to 1 mg. 



Spring pointed out that all previous workers had been imbued 

 with the conception of dirt as being of a fatty or oily nature, or 

 covered with a coating of such nature. In his experiments, therefore, 

 he carefully eliminated all fatty matter, wliich leaves the detergent 

 action of soap quite unimpaired. His striking and original experi- 

 ments dealt with purified lamp black, silica, alumina, and iron oxide. 



Spring's conception is that carbon promotes the hydrolysis of a 

 soap solution and forms a stable non- adhesive sorption compound 

 with the acid soap produced. Dirt upon a fabric he regards as being 

 combined with the fabric in an analogous way. Cleansing by soaji 

 is simply the formation of a sorjition compound dirt and soap in 

 place of the sorption compound dirt and fabric by direct substitution. 

 A more logical alternative is that of double decomposition in 

 which two sorption compounds are formed, fabric + soap and dirt 

 -\- soap. As a matter of fact it is often extremely difficult to 

 remove soap from a fabric after the operation of washing. Spring 

 points out that alcoholic solutions possess poor detergent power 

 because hydrolysis is not so great, this is not true if the alcoholic 

 soaj) is used in water. However, he found that whilst lamp black 

 took up acid soap, ferric oxide, sUicic acid, and cellulose take up soap 

 containing an excess of alkali, so that his results in some cases might 

 be more logically attributed to soap itself. The basic soaps of which 

 he speaks do not in fact exist. The iDoor detergent action of alcoholic 

 soaps on this view would simply be ascribable to the fact that in 

 alcohol the soap contains only traces of coUoid. 



Spring's two chief experimental results, are that lamp black is 

 carried through filter paper which, in the absence of soap, would 

 retain it completely, and, secondly, that the rate of sedimentation 

 is a function of the concentration of soap or, indeed, of alkali present. 

 Lamp black sedimented as fast in 2 per cent, soap solution as in 

 water, whereas in 1 per cent, solution it remains suspended for 



