Section III, 1918 [55] Trans. R.S.C. 



Periodic Precipitatipn 

 By Miss A. W. Foster, M.A' 

 University of Toronto 

 Presented by Professor E. F. Burton, Ph.D., F.R.S.C. 

 (Read May Meetîng, 1918.) 



One of the most interesting physical properties of colloidal parti- 

 cles in hydrosols (aqueous colloidal solutions) is the possession of an 

 electrical charge. When an emulsoid such as gelatine is added to the 

 hydrosol the particles still retain their peculiar charges, as is shown 

 by their motion in an electric field. On addition of electrolytes to the 

 hydrosol (without added gelatine), the charge on the particles is 

 lessened and coagulation of the particles takes place. This discharge 

 of the particles is thought to be due to the absorption by the charged 

 colloidal particles of the added ions, which bear a charge opposite in 

 sign to that on the colloidal particle. 



The purpose in undertaking the experiments detailed herewith 

 was to find whether colloidal particles in a matrix of solid gelatine 

 still absorbed ions in the same way. The phenomenon of Liesegang 

 rings was chosen in order to demonstrate the results. 



The Liesegang phenomenon is the name given to periodic precipi- 

 tates in some sort of gel produced by the diffusion of some reagent 

 placed upon the gel. The first mention of the phenomenon has been 

 attributed to Runge and also to Boehm, but it is due to Liesegang 

 that the subject has come into prominence. Chemical precipitates of 

 many different substances possessing a certain periodicity have been 

 obtained by the diffusion of different reagents in gels containing 

 various solutes. 



In the present experiments the Liesegang rings were formed by 

 the action of silver nitrate on a thin sheet of solid gelatine, which 

 has been impregnated with a small quantity of potassium chromate. 

 To a 4% gelatine solution was added potassium chromate to make up 

 1/200 gram molecular weight per litre. Two ces. of this solution 

 was poured on a glass plate (3^" x 4^4") kept perfectly level and 

 after the gel had set, that is, at the expiration of an hour or more, a 

 drop of 10 gram molecular weight per litre of silver nitrate solution 

 was dropped on the centre of the gelatine film. The resulting ring 



