23 



This is strictly true for only one concentration. Rather concentrated 

 soap solutions have an initial solidification temperature which is veiy 

 near the melting point of the fatty acid, dilute solutions of course fall 

 much below it, whilst, as we have found, highly concentrated soap 

 solutions come distinctly above it. No tenable explanation of this 

 behaviour has been put forward. It is probable that the soap fibres 

 consist of hydrated neutral coUoid and that the slight alkalinity of 

 the mother liquor is due to the particles of acid soaps resulting from 

 hydrolysis and which we have so often referred to. There are 

 particles in fact still visible under the ultramicroscope leading an 

 independent existence in the wet curd, and it is amusing to watch 

 a small particle performing violent BroAvnian movement up and do\\Ti 

 the length of a curd fibre from whose neighbourhood it often has 

 difficulty in escaping. This form of Brownian movement has not 

 previously been described. The coincidence with regard to melting 

 points refers of course, only to sodium soaps. 



It is worth while emphasizing that neither gelatinisation nor 

 formation of curd is identical in kind with the process of attainment 

 of equilibrium within the liquid sol. A true reversible equilibrium 

 is attained from both sides within the sol whilst it is supersaturated 

 with respect both to gelatinisation and formation of curd. 



Several authors have observed that formation of curd is accom- 

 panied by the evolution of heat. 



YLl.—The Effects of Additions. 



(a) Excess of Fatty Acid. 



In the case at least of the saturated fatty acids, the addition of 

 excess of acid results in the formation of acid sodium or potassium 

 salts which are insoluble. These acid soaps probably do not correspond 

 to any very definite chemical formula and as yet no complete chemical 

 analysis of such a specimen has been obtained from which, for example, 

 the amount of water in it could be deduced. As regards amount of 

 alkaU, their composition lies between that of the acid salt exemplified 

 by the acid sodium acetate, NaHAcj, and that of neutral soap. These 

 acid soaps do not lie within the region investigated from the stand- 

 point of the phase rule by Donnan and White, ^* who showed incidentally 

 that possibly no pure acid sodium palmitate NaHPg exists. 



In the case of sodium palmitate, we have found that addition of 

 palmitic acid to the solution causes a change in the conductivity 

 and also in the vapour pressure. Both conductivity and lowering 

 of vapour pressure when plotted against the amount of acid added, 

 are seen to diminish linearly to a small value at a point corresponding 

 roughly to HP • 2NaP. At this point the magnitude of the conductivity 

 and lowering of vapour pressure although small is definite and 

 independent of the original concentration of the sodium palmitate. 

 This behaviour clearly points to the formation of the insoluble acid 

 soap HP"2NaP, in equilibrium with its saturated solution. Further 

 addition of palmitic acid causes both the lowering of vapour pressure 

 and conductivity of the aqueous fluid to* become^^stiirsmaller, showing 



