710 Mr. A. S. Eve on the Properties of 



A plate, covered with a small quantity of radium, was placed 

 at a distance of 10 cms. from one extremity of a hollow metal 

 cylinder, along the axis of which was an insulated rod con- 

 nected with a gold-leaf system. There would thus be con- 

 siderable absorption of the a rays by the air traversed, the 

 capacity of the system would be large, and the ionization 

 effect at the far end of the cylinder would be uncertain in 

 character. 



In the present experiments 1 c.c. of water, containing 

 10~ 6 milligrams of radium, was slowly evaporated over the 

 bottom of a silvered glass flask so as to cover an area of 

 76 sq. cms. An ebonite cork was then carefully waxed to 

 the mouth of the flask so that it was hermetically sealed. The 

 ebonite supported a wire, sulphur bead, and gold-leaf system. 

 This electroscope was charged to 300 volts when a magnet 

 was brought near, so as to cause a small piece of magnetized 

 watch-spring to bridge the sulphur bead insulation. The 

 copper wire, as well as the silver coating of the electro- 

 scope, were then connected to earth. The natural leak of 

 the electroscope had previously been determined with care 

 for several days, and was not more than 5 per cent, of the 

 effect under measurement;. 



Voller found that 10~ 6 milligrams, distributed over 

 1*2 sq. cm., completely disappeared in 26 days. In my expe- 

 riment more than 40 days have elapsed since the maximum 

 was reached, and I have found no trace of disappearance, in 

 the case of 10~ 6 milligrams spread over 76 sq. cms.; and an 

 effect of 2 or 3 per cent, could have been detected with 

 certainty. 



Experiments were also made to measure the activity pro- 

 duced by various small quantities of radium. Professor 

 Rutherford kindly gave me some solutions of radium, and 

 the required amounts were distributed by evaporation over 

 small clean zinc or platinum dishes 4*9 .sq. cms. in area. 

 These were then placed in the bottom of a gold-leaf electro- 

 scope having a small natural leak. 



Attempts to measure 10~ 3 and 10 — 8 milligrams were not 

 successful, as the effects on the electroscope were respectively 

 too rapid and too slow, a result which might be expected 

 from theoretical considerations, if the activity is proportional 

 to the mass. In all cases the minimum values were observed, 

 when the emanation, if present, had been driven off by 

 evaporation, and the excited activity, if any, had time to 

 decay; that is, two or three hours after evaporation. 



A comparison of the results obtained by Voller and by 



