94 SCIENCE PROGRESS 



on the diffusion of true solutes, or else arrange themselves under 

 stress in such a manner that the passages available for diffu- 

 sion retain equal cross-sections in all azimuths with the axis 

 of compression. It is hardly possible to discuss this aspect 

 of the question until we have a profounder insight into the 

 origin of accidental birefringence in strained elastic gels, and 

 more particularly into the persistence of anisotropy long after 

 the removal of stress to which reference has been made (loc. 

 cit., i). 



The deformations obtainable in gelatin gels are comparatively 

 slight, especially if it is desired to keep them within the elastic 

 limit even for short periods of stress. Several attempts have 

 therefore been made to carry out similar experiments in gels 

 of vulcanised indiarubber. Such gels, obtained by vulcanising 

 rubber sols with various reagents, have been available for 

 some time and permit considerable deformations. The experi- 

 mental difficulties, however, caused partly by the volatility 

 of the dispersion medium and partly by the choice of a reaction 

 giving a sharp boundary, are so great that no conclusive results 

 have been obtained so far. 



