[foster] periodic PRECIPITATION 57 



a slight border, but the corresponding films containing copper gave a 

 much more marked effect. It may be that the agar solution reaches, 

 on account of its increased rate of solidifying, a state in which further 

 diffusion is prevented more quickly than the simple chromate agar' 

 solution, thus causing a concentration of the silver chromate precipi- 

 tate at the diffusion boundary. It would appear that the protective 

 action of the agar on the chromate is such that the presence of the 

 positively charged copper particle does not change the character of 

 the precipitate as it does in the gelatine. 



References: Liesegang: Zs. phys. chem. 88, 1914. 

 Ostwald: Zs. phys. chem. 32, 1897. 

 Morse and Pierce: Zs. phys. chem. 45, 1903. 



Stansfield: "Retarded Diffusion and Rhythmic Precipitation," Am. Jour. Sci., 

 Vol. XLIII, Jan. 1917, pp. 1-26. 



