15 



the homologous series is descended , with a definite break in the curve 

 between the hexoate and the acetate, once again indicating the 

 relatedness of the hexoate to the higher soaps. The apjiearance, 

 washing power, density, and conductivity curve of potassium hexoate 

 distinctly mark the beginning of that deviation from the behaviour 

 of the acetate, whicli rapidly and regularly increases through the 

 other members of the homologous series until it attains the typical 

 character of the higher soaps. 



It is worthy of note that the density of soap solutions does not 

 by any means conform with the conception of electro-striction, although 

 in these solutions there are almost as many ions and opposed electrical 

 charges as in the case of a typical electrolyte. 



(c) Surface Tension. 



Some of the matter which might have been discussed under this 

 heading will be referred to in paragraph 8 below, which deals with 

 detergent action, and further, the rather extensive subject of soap 

 films and bubbles will not be considered. Several years of further 

 investigation ^\'ould be required to answer all the questions which any 

 chemist would at once ask with regard to the surface tension of soap 

 solutions. The existing data are so isolated that it is still imj^ossible 

 to give a satisfactory broad survey of the subject, They refer, in 

 a few instances to surface tension against air, but most of the experi- 

 ments refer to surface tension at the interface with fatty or petroleum 

 oils. In many cases the results are affected to an unknowTi extent 

 by the carbon dioxide of the air, and frequently the composition of the 

 soap is extremely ill defined, and in the experiments, for example, 

 of Botazzi, the soap was rendered still more undefined by being 

 submitted to a process of dialysis. Another factor which can interfere 

 with the results is the rather ready oxidation of the unsaturated 

 substance like the oleate. It is well worth while determining not 

 merely the surface tension, but the actual amounts of adsorbed 

 substance, and still further the actual composition of the " soap " 

 in the meniscus ; that is, the amounts of alkali, fatty acid, and hydrate 

 water in the surface film. Lastly, an experimental difficulty which 

 should be taken into account is the insolubility of the saturated fatty 

 acids and of the acid soaps which separate out unless the solutions 

 are kept hot. 



Soap Solution — Air Interface. 



Sodium oleate solutions in contact with the atmosphere constitute 

 the only case in which the interfacial tension of the boundary between 

 soap solution and a gaseous phase, has received any serious considera- 

 tion. Plateau, Quinke, Marangoni, Lord Rayleigh, Milner, Hall, 

 Harkins, Perrin, and HUlyer may be referred to in this connection, 

 although some of the results refer to the chemical decomposition 

 of the soap by carbon dioxide, oxidation, and hydrolysis. By far 

 the most careful work is that of Harkins, Davies and Clark {Jour. Am. 

 ^Chem. Soc, 1917, 39, 586), who, like Hillyer {Ibid, 913, 25, 515), used 

 the drop weight or drop number method. These authors found that 

 when pure sodium oleate was added in increasing quantities to water 



