Colloids on Radioactive Products in Solution. 621 



elaborate paper, published by Miss Fellner, " Ueber die 

 Adsorption radioaktiver Substanzen durch Kolloide"*. 

 Miss Fellner carefully investigated the adsorption of certain 

 radioactive products by colloidal silicic acid. She found 

 that many products undergo considerable adsorption by 

 this hydrosol, so that it is possible to concentrate them 

 by this means. We may also note the researches of 

 Szillard f on the adsorption of UrX, and a new method 

 of concentrating this substance. Here we must also remark 

 that more than two years before my work and that of 

 Paneth was published, Szillard, whilst investigating the 

 nature of reactions of radioactive products, pointed out that 

 they may be considered as interactions of ions and electrified 

 suspensions, or, generally speaking, between electrified bodies 

 on each other. The mutual interaction between colloids is a 

 typical instance indeed of such a phenomenon. 



Starting from the assumption that radioactive products in 

 water form colloidal hydrosols, we have to expect that the 

 action of foreign colloids upon them will follow the well- 

 known laws + of interaction between colloids. 



Colloids, when of opposite sign, precipitate each other. 

 From the researches of Biltz § we know that this mutual 

 precipitation of colloids of opposite sign is limited by certain 

 conditions as to the quantities of both colloids present. For 

 every pair of colloids an " optimum " can be found corre- 

 sponding to the equalization of electric charges, which is 

 accompanied by complete precipitation. Thus there exists a 

 certain range of concentrations within which mutual pre- 

 cipitation takes place, the range which (e. g. in the case of 

 pigments) is the narrower || as the diffusibility of pigment is 

 less. If the degree of concentration of one colloid is far 

 outside the limits alluded to, no precipitation occurs, but 

 owing to ordinary and also to electrical adsorption the 

 colloid which is present in considerably smaller quantity 

 will gain (from the other) an opposite charge, so that both 

 colloids will travel in the same direction in the electric 

 field. 



The influence exerted by ordinary colloids on radioactive 

 products in solution has been examined by the writer, first 

 of all by means of observing the course of electrolysis. 



* Miss Felloer, Zeit. f. anorg. Chemie, lxxiii. p. 1. 

 t Szillard, Compt. Rend. 1909; Le Radium, 1909, p. 80, and 1910, 

 p. 360. 



X See, for example, Zsigmondy, ' Kolloidcheinie,' pp. 52, 220. 

 § Biltz, Ber. d. dent. chem. Ges. Band xxxvii. 1907, p. 1095. 

 || Zsigrnondy, loc. cit. p. 221. 



Phil. Mag. S. G. Vol. 27. No. 160. April 1914. 2 T 



