LXXT. Wafer Radioacfivity. Bij E. P. AdA3is*. 



rilHE t'oUoNvinp; is an account o£ some experiments made 

 1- to determine the nature ot" the radioactive gas found 

 in Camhridoe tap-water by Prof. J. J. Thomson. It will he 

 seen that this gas possesses properties remarkably similar to 

 the emanation of radium. 



Putherford and Curie have shown that the radioactivity of 

 the radium emanation decays according to the law 



J. — i^e , 



where X is a constant over a wide range of conditions. It is 

 therefore important to determine the rate of decay of the 

 water radioactivity and to see whether a similar law holds. 



A gold-leaf electroscope of the type devised by C. T. R.Wilson 

 has been used in mo^t of these experiments, but instead of 

 using sulphur insulation, a quartz rod, 10 cm. long and 

 2"5 mm. in diameter, was employed. The volume of the 

 electroscope was about 200 cc. The electroscope was charged 

 to 200 volts, and the movement of the gold-leaf read by 

 means of a microscope containing a scale in the eyepiece. 

 The gases to be studied w^ere admitted directly into the elec- 

 troscope, which was connected to a Toepler pump, through 

 a tightly-packed plug of glass-wool and a phosphorus-pent- 

 oxide drying-bulb. 



Rate of Decay of the Radioactivity. 



Water direct from a tap w^as passed through a copper tube 

 which was placed in a combustion furnace, so that the water 

 was thoroughly boiled on going through the tube. The gas 

 thus driven out of the water was collected in a bottle which 

 stood in a trough through which a stream of cold water con- 

 tinually circulated. In this way all the gas from a large 

 volume of water could be quickly collected, and tlie radio- 

 activity obtained in a very concentrated form. Sufficient 

 gas was collected to fill the electroscope to atmospheric 

 pressure. Readings of the rate of movement of the gold- 

 leaf were taken from time to time in order to get the rate at 

 which the radioactivity decayed. Innnediately after the gas 

 was introduced into the electroscope, the leak was at the rate 

 of 17 divisions per minute. This gradually increased for 

 about two hours, after which it began to decrease slowly. 

 The time required for the gold-leaf to move over the same 

 30 divisions of the scale was determined each time, so that 



* Communicated by Prof. J. .7. Tliomsoii, F.R.S. 



