Wellisch — Experiments on Active Deposit of Radium. 289 



of active deposit in the gas (i. e. gas activity) when the steady 

 state is established is inversely proportional to the coefficient of 

 diffusion (D) of the active particles in the gas. If therefore 

 we could determine the amount of active deposit in the gas 

 at any time we would be able to obtain the value of D. The 

 determination of this coefficient is of importance in the theory 

 of diffusion because the particles are of relatively large mass 

 and also because they are present in extremely small number. 



Before describing the method employed for determining 

 the amount of active deposit in the gas it is of interest to 

 record that when relatively small amounts of emanation were 

 employed the gas activity was found to be a definite fraction 

 of the total activity. With larger amounts of emanation, 

 however, variations of such an exceptionally large order 

 were found to occur in the values of the gas activity that it 

 became of interest to make a systematic investigation. The 

 results obtained in this connection are given in the present 

 section ; the determination of D by the use of relatively small 

 amounts of emanation is described in the next section. 



In the experiments now to be described it was not found 

 necessary to determine the total amount of the active deposit 

 present in the gas. The experimental procedure may be 

 briefly described as follows : approximately 2-J- millicuries of 

 radium emanation* mixed with air was introduced through 

 drying tubes and cotton wool into the cylindrical testing 

 vessel, which was closed at the top with a rubber stopper, 

 no central rod being present. Several hours and occasionally 

 a few days were allowed to elapse in order that any dust 

 present in the gas might have a chance to subside. The 

 outer case was connected to any desired potential, but as there 

 was no inner electrode, the electrical conditions inside the 

 cylinder were not disturbed. When conditions were steady 

 an electrode permanently connected to earth was introduced 

 for one minute and then withdrawn, the cylinder being again 

 closed by the rubber stopper. It was demonstrated repeatedly 

 during the course of the investigation that less than 1 per cent 

 of the amount of emanation escaped during the process of 

 introducing and withdrawing the central electrode ; the 

 decay of the emanation was examined from day to day and in 

 this way the loss could be estimated. It was found that the 

 most convenient method of comparing the activities of rods 

 which were introduced under different conditions was to 

 measure the activity at any convenient period after removal 

 and then to calculate the activity corresponding to the lapse 

 of a definite period from the time when the rod was originally 



* Kindly supplied by Professor Boltwood. 



