234 



Mr. Eve and Dr. Mcintosh on the Radii 



After a definite period, usually about a week, the solution 

 was thoroughly boiled, and the expelled air and emanation 

 were collected over water. These were drawn into an electro- 

 scope which previously had been almost exhausted of air by 

 a water-pump. Three hours later, when the active deposits 

 had nearly reached maximum activity, the movement of the 

 gold-leaf was measured by a microscope with a graduated 

 eyepiece. Since the emanation in the flask increases to half 

 its maximum value in about 3*8 days, it is easy to calculate 

 the maximum from the amount measured after any definite 

 period. It was found by experiment that the emanation was 

 not lost to an appreciable extent by absorption when collected 

 over water, if placed in the electroscope without delay. 



The electroscope, as shown in the figure, consisted of a 



to Water Pum p. 



filter-flask silvered inside, Strips of tinfoil moistened with 

 phosphoric acid connected the silver coating to the earth 

 wire. The flask was closed by a rubber stopper to which the 

 usual gold-leaf arrangement was attached. It could be ex- 

 hausted by a water-pump to a pressure of 1 or 2 cms. of 

 mercury. Air and emanation could be admitted through a 

 capillary-tube and a bulb of phosphorus pentoxide. The 

 natural leak of the electroscope was 3'9 divisions an hour, 

 and this remained remarkably constant, provided the electro- 

 scope was exhausted and refilled daily. The electroscope 

 was not influenced by exterior electrification, nor by atmo- 

 spheric conditions. 



Blank tests were made involving all the chemicals and all 

 apparatus used in the experiments. No radioactive matter 

 could be detected by these tests. 



The electroscope was standarized by inserting the air and 

 emanation boiled from a flask containing l'57xl0~ 9 gram 

 of radium, and the resulting effect was a movement of 10*3 

 scale- divisions per minute after 3 hours. 



The standard solution was that used by Rutherford and 



