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XLI. On Magnetic Doable Refraction of Gold Colloids, 

 By Yngvb Bjorxstahl *. 



[Plate XIII. j 

 Introductory. 



WE know that solid and fluid substances, when placed 

 in an electric or a magnetic field, acquire double 

 refraction. The former phenomenon is named after the 

 English physicist Kerr, the latter is the so-called Cotton 

 effect. 



Apart from some observations on suspensions f, an action 

 of electric strain on colloids was first established by 

 Diesselhorst, Freundlich, and Leonardt J in the case of 

 V 2 5 -sol, but other colloids examined did not show any 

 effect. Bergholm and I have shown that an electric field 

 induces double refraction in gold and silver sols as well §. 



Majorana || and Kerr IF were the first to observe that a 

 solution of " Fer Bravais," when subjected to a magnetic 

 field, exhibited properties similar to those of a uniaxial 

 crystal. Schmauss** drew attention to the fact that it was 

 a phenomenon characteristic of certain colloids and suggested 

 that it was due to an orientation of particles. Cotton and 

 Mouton ft made a careful examination of the phenomenon, 

 also using the ultramicroscope. As the result of an extensive 

 series of experiments, they inferred that the ferric oxide and 

 the iron colloids consisted of aeolotropic particles. Later 

 Diesselhorst, Freundlich and Leonardt it observed an 

 analogous effect in Y 2 5 -sols. 



A disperse system consists of two phases belonging to any 

 one of the three states of aggregation. I deal especially with 

 colloidal liquids, in which at least one of the constituents is 

 fluid. When studying magnetic double refraction in such 

 systems, it is of importance to take care that the so-called 

 " direct effect," the Cotton phenomenon, is eliminated, in 

 order to obtain only the double refraction characteristic of 

 the colloid. As a rule there is no difficulty about this ; the 



* Communicated by the Author. 



f Ch. Chandler, C. R. cxxxrii. p. 248. 



X Elster-Geitd-Festschrift, 1915, p. 476. 



§ Phys. Zeit. xxi. p. 137' (1920). 



|| Rendic. R. Accad. Lincei, xi. i. p. 374. 



% Brit. Assoc. Report, 1901, p. 568. 



** Drude's Ann. d. Phys. (4) xii. p. 186 (1903). 



tt C. R. cxli. pp. 317, 349 (1905). 



XX Loc. cit. 



