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LXIL Double Refraction in Colloids produced by Electric 

 Endosmose. By H. W. Malcolm, M.A., B.Sc, Ph.D., 



Carnegie Fellow of Aberdeen University* '. 



Introduction. 



DOUBLE refraction in colloids has been the subject of 

 numerous investigations ; a summary is given hj 

 G. Quincke, Ann. d. Phys. x. p. 482 (1903), and Ann. d, 

 Pliys. xiv. p. 849 (1904). The latter paper deals with double 

 refraction in gelatine produced by soaking. and shrinking. 

 It is shown that on cooling warm gelatine solution an optically 

 isotropic jelly is obtained, built up of cells with walls of an 

 oily nature, *. e., showing surface-tension with the surrounding- 

 liquid. Prisms of this jelly when bent become doubly re- 

 fracting, positively at places of dilatation, negatively at places 

 of contraction, with optical axis parallel to the stress direction, 

 as in the Case of bent glass strips. 



When an electric current flows through a jelly consisting 

 of oily, water-poor liquid A with water-rich content B, one 

 would expect, through the contact electricity at the places of 

 contact of the heterogeneous substances, a transference of liquid 

 along the cell-walls to take place, and simultaneously a dis- 

 placement of the still-liquid, viscous cell-walls in the opposite 

 direction, producing a state of strain and consequent double 

 refraction. 



Gelatine in Capillary Tubes. 



A capillary tube 30 cms. long, outer diameter 1*2 mm., 

 was filled with a 10 per cent, solution of gelatine and the 

 lower end sealed off. Several such tubes were kept for a 

 quarter of an hour in a long test-tube full of water at 100°, 

 and slowly cooled, so that the gelatine solidified without strain 

 and without double refraction ; finally the upper end was 

 sealed off too. As required, pieces 4*5 cms. in length were 

 cut off and cemented to a glass slide. Over the ends of the 

 tube were placed cover-glasses, supported by small empty 

 pieces of the same tube. A drop of water was brought under 

 each cover-glass, into which dipped platinum wires 0*5 mm. 

 thick as electrodes ; in this way disturbance due to gas-bubbles 

 was avoided. 



The slide was laid on the table of a microscope and could 

 be examined between crossed nicols with different magnifying 

 powers ; between the prisms a wave-length selenite plate 

 could be inserted, showing the red of the first order. The 



* Communicated by the Author. 



