JBumstead and Wheeler — Radio-active Gas. 



99 



it was tested after each reading by changing the applied poten- 

 tial by 10, 20, and 30 volts and noting the change in deflection ; 

 the readings were then reduced to a standard sensitiveness. 

 We are indebted to Mr. H. M. Dadourian for his assistance in 

 making these numerous and somewhat tedious observations. 



The resemblance of this gas to the emanation from radium, 

 in respect to the period of initial rise of activity (which is 

 due to the deposition upon the walls of the electroscope of 

 " induced " or " excited " activity) and also in respect to the 



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rate at which the activity of the gas itself decays, is so close 

 that we could not hope to detect any differences between the 

 two by means of observations with the electroscope. Accord- 

 ingly, we had recourse to observations with a quadrant elec- 

 trometer, and during the autumn, the earth-gas was directly 

 compared by means of the same apparatus with the radium 

 emanation, with regard to the rise of excited activity, the 

 decay of the activity of the gas itself, and the rate of diffu- 

 sion through a porous plate of unglazed porcelain. The elec- 

 trometer used was of special construction, made after our 

 design by Mr. L. J. Barrett, mechanician of the Sheffield Scien- 

 tific School ; it proved to be very steady and convenient for 

 the present purpose, and may perhaps be described later, after 

 its behavior under varied conditions has been more fully inves- 

 tigated. When the needle was suspended by a quartz fiber of 

 very moderate thinness, the sensitiveness with 100 volts on the 

 needle was about 250 cm per volt, the scale being one meter dis- 

 tant from the mirror. As this was sufficient for our present 

 needs, no attempt to increase it by using a finer suspension was 

 made, particularly as we wished to make observations, in some 

 cases, in as rapid succession as possible and a very large period 

 of the needle was, therefore, undesirable. 



