172 Prof. Thomson, On the Existence of a 



On the Existence of a Radio-active Gas in the Cambridge Tap- 

 water. By Professor Thomson. 



[Read 4 May 1903.] 



When Cambridge tap-water is boiled the air given off is mixed 

 with a radio-active gas. The existence of this gas is easily 

 demonstrated by electrical means, for if the air expelled by 

 prolonged boiling from about 10 litres of water is introduced into 

 a closed vessel whose volume is about 600 c.c. the amount of 

 ionization in the vessel (as measured by the saturation current) is 

 increased five or six times. When the water has once been well 

 boiled the gas expelled on any subsequent reboiling is not 

 appreciably radio-active. The gas can also be extracted from 

 water at the temperature of the room by vigorously bubbling air 

 through it ; the air as it bubbles through the water gets mixed 

 with the radio-active gas and carries it along with it. When 

 water which has been treated in this way is boiled no radio-active 

 gas is given out ; nor is the gas given off when air is bubbled 

 through water which has been well boiled. 



The gas extracted in this way from the water retains its radio- 

 active properties after bubbling through strong sulphuric acid, or 

 caustic potash after passing over red-hot copper, or through a 

 narrow platinum tube kept at a white heat : it does not seem 

 appreciably affected when sparks are passed through it. 



The gas can diffuse through a porous plate, and by comparing 

 its rate of diffusion with that of C0 2 through the same plate its 

 density can be determined by Graham's law : preliminary measure- 

 ments of this kind indicate that two different gases are present, of 

 which one has a density about twice, the other between six or 

 seven times that of C0 2 . The gas obtained by boiling the water 

 always diffused faster than that procured by bubbling air through 

 the water ; it seems possible that in the latter case the gas may 

 get loaded with water-vapour to a greater extent than in the 

 former. 



A negatively electrified surface exposed to the gas becomes 

 radio-active, the induced radio-activity dying away to half its 

 value in about 45 minutes. Mr Adams has shown that a positively 

 electrified surface also becomes radio-active when exposed to the 

 gas, though to a smaller extent than if it had been negatively 



