THE COLLOIDAL STATE 



99 



The work of Bachmann (1912) and of Zsigmondy (1913) on the formation of 

 gels, as observed with the ultra-microscope, is of interest. Most of the work was 

 done with pure soaps and is illustrated by Fig. 43. It is well known that a fairly 

 strong hot solution of sodium or potassium stearate or palmitate sets to a more or 

 less transparent, tenacious jelly, when it cools. This usually changes later into an 

 opaque, white, friable mass. The former corresponds to a fine felt-work, as seen 

 under the ultra-microscope (D and F of the figure) ; while the latter is obviously 

 crystalline (D in the figure). The structure of the gel, as first formed, shows a 

 strongly polarised cone of light and is, therefore, of an extremely fine degree of 

 heterogeneity, much finer than the foam structures described by Blitschli. As 

 cooling proceeds, the particles become larger, Brownian movement is easily seen 

 (A). These particles continue to increase in number, obstruct one another in 

 movements, and suddenly form threads, which are said to have a "crystalline" 



B 



D 



E 



F 



H 



FIG. 43. DIAGRAMS OF ULTRA-MICROSCOPIC APPEARANCE OF SOAP GELS IN PROCESS 



OF FORMATION. 



A, B, C, D, E, Stages of gelation and crystallisation of 5 per cent, potassium stearate in water. Enlarged about 

 200-300 times. 



F, Jelly of 10 per cent, sodium oleate. Felt-work obtained by dissolving away the finer threads. Cardioid 



condenser. Magnified about 140 times. 



G, Jelly of 5 per cent, sodium palmitate. Needle-like fibres. 



H, Crystals of potassium stearate from watery solution of about 10 per cent. Final state. Magnified about 

 300 times. 



(After Bachmann.) 



appearance (see F and G), and result in the production of a felt- work. After a 

 time, this felt-work changes into distinct separate crystals (E and H). Whether 

 the first particles are to be regarded as " micellae," in Niigeli's sense, that is, as 

 aggregates with crystalline properties, is a matter for argument. It is clear, 

 however, that the vectorial forces, which ultimately result in the formation of 

 distinct crystals, must be always present, but apparently require time for action. 

 The ultra-microscopic particles, probably of a crystalline form, at first separate 

 out arranged in threads and networks. 



An important point in regard to the nature of the two phases is that 

 Bachmann found that the " inter-micellar " fluid, in the case of a gel of 1 per 

 cent, sodium palmitate, contained 0-06 per cent, of the salt. 



IMBIBITION 



Many emulsoids, after being dried, are capable of taking up again large 

 quantities of water, without actually forming liquid solutions, such as ordinary 

 hygroscopic substances, calcium chloride, for, example, do. 



