Compounds of Short-lived Radioactive Element*. 131 



The decay-curves obtained showed that the loss of: activity 

 was due almost entirely to solution of actinium B in the 

 water. Only a very small quantity, if any, of actinium C 

 was dissolved. The platinum surface had not suffered any 

 visible change by this treatment. It appears, however, that 

 a thin layer of acid is formed on the surface of the metal by 

 the treatment, because the activity can be relatively easily 

 removed from such a piece by rubbing. Incidentally it may 

 be mentioned that actinium B is easily dissolved by acids 

 even in very dilute solution. A concentration of *001 

 normal hydrochloric or sulphuric acid is sufficient to 

 dissolve in a minute more than 95 per cent, of the actinium 

 B off a quartz surface, and more than 80 per cent, off a 

 platinum one. Hardly any actinium C is, however, removed 

 by this treatment. 



It is obvious from the experiments described that the 

 nature of the surface and of the surface-layer is an im- 

 portant but indefinite factor in the volatilization of minute 

 quantities. For this reason experiments were made along a 

 different line in which this factor is partly eliminated. 



II. Condensation Experiments. 



In the experiments which will now be described, the 

 active material was first volatilized completely and the 

 amount and composition of the material condensed on 

 surfaces at definite temperatures investigated. It is to be 

 expected that the active material would begin to condense 

 as soon as it reaches a surface whose temperature is less than 

 the volatilization-temperature. By this method volatility 

 can be measured independent of any effect due to the 

 surface-layer, since the condensation takes place at high 

 temperatures. 



The experimental arrangement was as follows : — The 

 porcelain tube of an electric furnace placed vertically up- 

 right was closed at its lower end by asbestos and kieselguhr. 

 Inside this was placed a quartz tube 32 cm. long and 1*1 cm. 

 wide, closed at its lower end. The furnace was wound with 

 a platinum wire in such a way that the temperature was 

 highest at its lowest point and gradually decreased towards 

 the other end. The active material was volatilized from a 

 platinum surface, and was condensed on a thin long mica 

 strip placed inside the quartz tube. The temperature of 

 this strip at different points was determined by means of a 

 thermocouple. 



As soon as the furnace had reached a steady state of 

 temperature, a small piece of very active platinum wire was 



K2 ' 



