24 



that the acid soap becomes more and more insoluble as the amount 

 of acid in it increases beyond the composition mentioned. These 

 " solutions " appear as translucent pastes or jellies at 90°. On cooling 

 silky fibrous crystals of acid soaps may separate. More dilute solutions 

 froth very readily. 



(b) Excess of Sodium Hydroxide. 



Most of the phenomena occurring upon the addition of caustic soda 

 or other electrol3^es, which ultimately result in the quantitative 

 salting out of the soap from the solution, will be discussed in Part II., 

 which deals with the technical colloidal chemistry of soaps, their 

 solutions, gels, and curds. 



Rarely are homogenous solutions of soap to be met with during 

 the process of manufacture, since these would be of prohibitively 

 great viscosity. Usually, as has been found in this laboratory, the 

 soap is in the form of an emulsion of two soap solutions, whether 

 sol-sol, sol-gel, or sol-curded gel. The experimental evidence will be 

 ]')ublished elsewhere. 



A number of theoretical points also must be reserved for forth- 

 coming publications. However, it should be mentioned here that 

 for the greater part these additions do not actually combine with 

 the soap, but drive back its dissociation or displace the numerous 

 equiUbria involved. 



VIII. — Detergent Action of Soaps. 



The brilliant work of a number of such investigators as Hillyer 

 (1903) and Spring (1908) has shown conclusively that the detergent 

 power of soaps is due to their colloidal nature, and it is only influenced 

 by their chemical constitution in so far as this affects their behaviour 

 as colloids. 



In commerce soaps are usually bought and sold upon their 

 appearance and texture after assurances have been given with regard to 

 their classification as curd soaps, tallow rosin soaps, cleanser soaj)s, &c. 

 There are no universally accepted quantitative standards to which 

 soaps are referred with regard to their washing power. It is convenient 

 to give a list of all the cognate experimental methods hitherto 

 published as having more or less bearing upon this question : — 



(1) Measurement of surface tension against air by capillary tubes 



or by drop members or by bubbling or by measuring the 

 amount of froth produced under definite conditions. 



(2) The measurement of surface tension against oil or paraffin oil 



or benzene by drop numbers or measurement of emulsifica- 

 tion. 



(3) Measurement against carbon or other powders by measuring 



rate of sedimentation or protective action in filtration. 



(4) Protective action as measured by gold numbers. 



(5) Direct washing experiments with sjoeciaUy soiled clothes under 



controlled conditions of true temperature and concentra- 

 tion. (Zhukov and 8hestakoi\ Chem. Ztg., 1911, 35, 1027.) 



