22 



and life histroy of the various structural elements, have shown that 

 soap curds consist invariably of fine fibres. 



In the case of all sodium soaps these may be many centimetres in 

 length, and may be straight or characteristically curled but they are 

 never of greater thickness than about 1 micron. Most of them are of 

 ultramicroscopic diameter, and the thicker ones may be always or 

 usually merely parallel bundles of finer ones These fibres are often 

 so fine that shorter or unattached fibres exhibit B^o^vnian movement 

 when the medium is not too viscous. These curd fibres constitute 

 the only mechanical structural element of sodium soap curds and 

 represent the stable condition of such curds even after the lapse of 

 years. 



The potassium soaps are somewhat more complicated and the 

 phenomena require more time for development. Upon slow cooling, 

 minute V-shaped twin fibres appear of about a few tenths of a micron 

 in diameter, but never more than a few hundredths of a millimetre 

 in length as compared with the inch-long fibres of sodium soaps. These 

 twin fibres hoVever, do not constitute the stable solid phase in 

 potassium soap solutions for, on standing for a few days, they gradually 

 become replaced by thin irregular leaflets or crystals less than 1 micron 

 in thickness. 



The curd fibres of sodium soaps may scarcely be called crystals, 

 nevertheless they constitute a sufficiently definite phase to exhibit 

 some of the properties of crystals. For instance, they appear to 

 exhibit a definite solubility at each temperature. A curd of sodium 

 soap therefore in general consists of a felt of curd fibres in which 

 is enmeshed an aqueous liquid containing, according to the tem- 

 perature, traces of soap and alkali, the product of hydrolysis, or 

 perhaps large amounts of soap sol or gel depending upon age, 

 previous history and, of course, temperature. 



At the temperature of initial solidification only a few fibres are 

 formed, the bulk of the soap remains in the solution which therefore 

 exhibits a practically undiminished vapour pressure and conductivity. 

 As the temperature is lowered, the solubility-of the curd fibres rapidly 

 diminishes until the enmeshed liquid consists chiefly of water and its 

 vapour pressure and conductivity behave accordingly. Throughout 

 this range of temperature the stable condition of the soap solution 

 is the formation of the appropriate amount of curd fibres with 

 enmeshed gel. The definite solubility of the curd fibres at any one 

 temperature is evinced by the fact that the conductivity of a well-aged 

 curd is approximately independent of the concentration of the original 

 soap. 



It follows from the above conception based upon direct measure- 

 ments that different concentrations of any one soap cannot solidify 

 at the same temperature, but this depends upon the temperature- 

 conductivity curve of the curd fibres. The lower the concentration 

 of the soap, the lower the temperature of initial solidification. 



It is a remarkable fact to which Krafft and Wigelow^' called 

 attention, that there is a certain parallelism between the initial 

 solidification temperature of a sodium soap solution and the melting 

 point of the pure anhydrous fatty acid from which the soap is derived. 



