4 STUDIES IN GELS 89 



ject to tensions or pressures; the designation is, therefore, double 

 refraction due to tension or tension double refraction. This phenomenon 

 accompanies elastic deformation (photo-elastic effect), and elastic de- 

 formability is a condition for its occurrence. Since, according to 

 definition, gels actually do possess this property (Table X, p. 74), 

 effects of this kind are to be expected in gels exposed to stress. The 

 tension double refraction is usually positive with respect to the 

 axis of deformation, while that due to pressure is usually negative. The 

 effect is most pronounced in isotropic gels (e.g., strain-free gelatin), 

 but is of course also observed in gels which are anisotropic by nature 

 if these are exposed to tensions, in which case it is superposed on the 

 pre-existing textural and intrinsic double refraction. On removal of the 

 stress, the tension double refraction must disappear, as with every 

 really elastic phenomenon. If it does not, the object has been plastically 

 deformed. The photo-elastic effect is due to the deformation of elec- 

 tron orbits in the material concerned ; the distances between the atoms 

 in this material are slightly increased or decreased. In cubic crystal 

 lattices insignificant changes in atomic distances cause considerable 

 optical anisotropy (Wiener, 1926b). 



Orientation double refraction. The junction bonds in a gel being seldom 

 very strong, they easily yield to the forces apphed. The elastic deforma- 

 tion is then followed by a re-orientation of the micellar strands, thus 

 intensifying the intrinsic and textural birefringence of the gel. For this 

 reason the optical phenomena in gels exposed to stress are often very 

 comphcated. The difference between the double refraction due to 

 tension and that due to orientation is most obvious when these 

 phenomena are different in sign, as for example in the basic experi- 

 ments of Ambronn (1889) with cherry gum. For, when cb^rry gum 

 is stretched, the transient, weakly positive double refraction resulting 

 from the tension is followed by a negative double refraction due to 

 the orientation of the micellar texture. 



D'stribution of orientations. In a stretched gel, the directions of the 

 micellar units are spread about the reference axis according to a com- 

 plicated distribution function (Kratky, 1933, 1938). The majority 

 of micellar strands enclose small angles with the direction of the 

 stretch, and only few of them enclose large angles with this direction. 

 The distribution function depends on the degree of stretch. If this 

 strain is unknown, however, an idealized scheme of the distribution 



