Active Deposit oj Actinium in an Electric Field. 763 



mental difference between the method adopted by Walmsley 

 and that followed in the present investigation. In Walmsley's 

 experiments no direct measurement of the activity of the 

 outer electrode (cylindrical vessel with the air-inlet on the 

 side) was made ; the total activity was assumed to be equal 

 to the sum of the activities obtained when the central rod 

 was first made the cathode and then the anode. It will be 

 shown later that this assumption is not justifiable. In the 

 present research the activities of both the case and the 

 central electrode were obtained by direct measurement. 



II. Experimental Procedure. 



The source of the actinium emanation employed was a 

 preparation of actinium, which Professor Boltwood kindly 

 placed at my disposal. 



The air current, which carried the emanation into the 

 testing-vessel, was produced by means of an ordinary water- 

 blower. This gave the desired high velocities of the air 

 current, which could not be conveniently obtained by means 

 of a Mariotte's bottle or a gasometer. In order to obtain 

 constancy of flow three regulators were installed in the 

 system : a water-pressure regulator, in series with the 

 water-blower, and two gas-pressure regulators along with 

 several large capacities. With these appliances the varia- 

 tions in the pressure were reduced to about 1 per cent. 



The air current was conducted through a concentrated 

 solution of potassium hydroxide, to eliminate carbon dioxide, 

 through two bottles of concentrated sulphuric acid to dry it, 

 and through a train of two or more tubes containing P 2 5 , 

 plugged at each end with glass-wool, to absorb any residual 

 minute traces of moisture. The perfectly dry condition of 

 the last P 2 5 tube indicated that no trace of moisture was 

 left in the air current. This dry air current was then passed 

 through the tube containing the actinium preparation, which 

 was plugged at both ends with cotton-wool. Finally, the 

 emanation was led into the testing-vessel through another 

 cotton-wool plug- 



The vessel used in the experiments consisted of a brass 

 cylinder with an insulated central electrode sprung into a 

 split brass rod and very easily detachable from it. This rod 

 in turn was rigidly fixed in the tapered ebonite plug P, 

 which fitted closely into the guard-ring E. By employing 

 sealing-wax and stop-cock grease all the joints, shown in the 



3D2 



