Boltwood — Salts of Radium. 411 



solution in each of several flat, glass dishes 50 mm in diameter 

 and 9 mm in depth. The time taken for the evaporation was 

 about three hours, and the salt was deposited in a fairly uni- 

 form and extremely thin layer on the bottom of the dish. The 

 freshly prepared films were introduced into an electroscope 

 and their activity measured. 



The electroscope in which the solids were measured con- 

 sisted of an ionization vessel of sheet metal 14 cm in height, 

 circular in cross-section, 15 cm in diameter at the top and 

 bottom and 19 cm in diameter at a point midway between the 

 top and bottom. A circular aluminium plate, 7\5 cm in diam- 

 eter, was supported 9 - 5 cm from the bottom by a vertical, 

 insulated, brass rod, which extended through the top of the 

 ionization vessel and carried a small gold leaf at its upper 

 extremity. This gold leaf was surrounded by a metal case pro- 

 vided with small mica windows. The insulated plate could be 

 charged at will to a potential of about 400 volts from a small 

 storage battery, and the leak of the charge from the plate 

 determined by the fall of the gold leaf which was observed 

 through a microscope with a transparent scale in the eye-piece. 

 The lower half of the ionization chamber could be swung to 

 one side about a pivot without disturbing any other part of the 

 instrument, permitting the placing on the bottom of the ioniza- 

 tion chamber of the dishes containing the substances to be 

 tested. When in its normal closed position, the bottom of the 

 ionization chamber was held firmly by three small spring 

 clamps. The entire metal case of the electroscope was electri- 

 cally connected to earth, as was also the positive pole of the 

 battery. The insulated plate was in all experiments charged 

 negatively. 



The initial activity of the films having been determined, 

 they were placed in an air-tight bell-jar over fresh, concentrated 

 sulphuric acid and allowed to remain in this desiccating atmos- 

 phere during the intervals between the different measurements. 

 The experiments were conducted during the winter months in 

 a building heated by steam, and measurements were made only 

 on clear, cold days when the atmosphere in the laboratory con- 

 tained a minimum amount of moisture. When a measurement 

 was to be made, the dish carrying the film was removed from 

 the bell-jar, placed immediately in the electroscope and the 

 measurements carried out as rapidly as possible. The dish was 

 then replaced in the bell-jar and another one removed and 

 measured. 



The initial activities of the several films and the activities 

 after the lapse of the stated times is given in the following- 

 table in terms of the fall of the gold leaf in divisions per 

 minute. The readings in each case are corrected for the 



