628 Prof. T. Grodlewski on the Action of 



at identical curves it was necessary, especially in the case of 

 low concentrations, to keep all external conditions of indi- 

 vidual experiments exactly alike. Similar facts have been 

 observed in dealing with the precipitation of ordinary 

 colloids *. 



Leaving aside these details we may conclude by saying 

 that ferric oxide, and generally speaking all positive colloids, 

 when added in relatively small amount, conduce to precipita- 

 tion of BaA and RaO, which are among negative colloids ; in 

 higher concentrations positive colloids adsorb them, thus 

 inverting their signs. This action is therefore exactly oppo- 

 site to that of negative hydrosols, as was of course to be 

 foreseen from theory. 



5. Precipitation of Radioactive Products with Colloids. 



The experiments heretofore described show that relatively 

 small quantities of colloids are able to adsorb all the products 

 contained in a solution. The question now arises : In how 

 far are the compounds of colloids with radioactive products 

 stable ? In order to elucidate this point the following 

 experiments were made. 



To 16 cm. 3 of water saturated with radium emanation 

 a few milligrams of colloidal arsenious sulphide, platinum or 

 gold were added; the latter was obtained either by Bredig's 

 method or by that of Zsigmondy f ; after which the colloids 

 were precipitated, using a small amount of aluminium sul- 

 phate solution. 



The precipitate collected on a small filter w r as tested as to 

 activity immediately after desiccation. It was found that all 

 the products (RaA, RaB, and BaC) were present on the 

 filter ; and further, that by means of one such single pre- 

 cipitation it was possible to separate the products almost 

 quantitatively, so that the filtered liquid contained scarcely 

 any product. 



It was likewise possible to collect by the same method the 

 radioactive products on positive hydrosols e.g. on ferric 

 oxide. A greater amount of this hydrosol, however, was 

 required in order to effect the adsorption ; besides, this col- 

 loid owing to greater voluminousness is much more slowly 

 filtered ; hence such colloids as gold or platinum are more 

 suitable when it is desired to concentrate radioactive 

 products. 



By the use of these colloids we can thus easily concentrate 

 powerful activity on minute quantities (a few milligrams) of 

 precipitated hydrosols. This method proves very convenient 



* See e. g. W. Biltz, loc. cit. 

 t Zsigmondy, loc. cit. p. 94. 



