Colloids on Radioactive Products in Solution. 6'M 



practically inactive. This last observation is o£ importance 

 iu so far as it clearly shows that the phenomenon with which 

 we have to deal is due essentially to the precipitation of 

 radioactive products in the colloidal state, and that it does 

 not depend on a process of adsorption. At high concentra- 

 tions of acid , indeed, we have no longer a colloidal, but rather 

 an ionic solution, in which no precipitation occurs. 



In the latter case, the products are prevented from being 

 precipitated on the filter by being dissolved in a strong acid. 

 We are able, howeA^er, to maintain the products in colloidal 

 state without allowing them to remain on the filter. This 

 may be done by transforming all radioactive products inlo 

 negative colloids. If to a solution a sufficient amount (a feAv 

 milligrams) of a negative colloid be added, e.g. of gold, 

 platinum, orarsenious sulphide, the radioactive products pass 

 freely through the filter, which shows no activity at all. 

 The addition of 1 cm. 3 of y 1 ^ normal potassic citrate reduces 

 similarly the activity of the filter to less than 10 per cent. ; 

 the effect of the citrate, as is known, is to change the sign of 

 positive hydrosols, transforming them into negative ones ; 

 and the latter are not precipitated. 



Experiments were also made to examine whether the 

 precipitation of colloids which form the positive hydrosols 

 could be shown by observing the ascent of water saturated 

 with emanation in sheets of filter-paper. In the immersed 

 part of the paper, RaB and RaC are found in the same rela- 

 tive amount as on the filters. When the sheets are immersed 

 for a long time, we find also RaB and RaC in the parts lying- 

 above the surface of the liquid in its immediate proximity. 

 At higher levels, however, only pure RaC was noticeable. 

 The explanation of this is simple. Negative hydrosols only, 

 i. e. RaA and RaC, can ascend in the paper. Of these, RaA 

 can travel through a small distance only, because, owing to 

 the short period of its transformation, it becomes soon trans- 

 formed into RaB, which is precipitated. In consequence the 

 RaC is the only one which ascends somewhat higher. 



We can collect the radioactive products by filtration on 

 glass-wool just as well as on paper. 



The experiments so far described supply us with a new 

 and strikingly simple method of concentrating radioactive 

 products. For this purpose the products require only to be 

 filtered. If, as frequently happens, they are to be collected 

 on the smallest possible quantity of foreign substance, we have 

 only to burn the filter, which reduces then to a negligible 

 amount of ash. And since the process is based on a property 

 belonging to all positive colloids in general, it follows that 



