Radio-active Gas in the Cambridge Tap-water. 173 



electrified ; an unelectrified surface does not become radio-active. 

 In this respect the gas differs from the emanation from radium, 

 which according to Rutherford produces much more induced 

 radio-activity in an unelectrified surface than in a positively 

 electrified one. 



The rate of diffusion through a porous plate of the gas obtained 

 by bubbling air through distilled water containing a trace of 

 radium is not the same as that of the gas got by bubbling through 

 tap-water. 



If the gas is confined in a closed space its radio-activity slowly 

 diminishes. Mr Adams found that the gas contained in a vessel 

 of about 300 c.c. capacity lost when not exposed to an electric field 

 about 5 °/o °f its activity in 24 hours, under a strong electric field 

 the rate of loss was doubled. Water drawn from the tap and left 

 exposed in a bucket for a fortnight gave off very little of the gas 

 when subsequently boiled. I have not found any of the gas in any 

 of the numerous samples of rain and surface water which I have 

 tested. 



Professor Dewar (to whom I am greatly indebted for assistance 

 and advice) was kind enough to subject the gas obtained by boiling 

 the water to treatment by liquid air. Two samples were treated, 

 one containing about 80 litres of gas, obtained from the coppers of 

 the Star Brewery, Cambridge, by the kindness of Mr Armstrong 

 (to whom I wish to express my thanks), was passed slowly through 

 a bath of liquid air and samples of the emergent gas collected : 

 this on testing was found to have no radio-activity, though it was 

 strongly radio-active before passing through the liquid air : it is 

 evident therefore that at the temperature of liquid air the radio- 

 active gas is frozen out. The other sample of 20 litres prepared 

 in the laboratory was actually liquefied : the liquid was then 

 allowed to boil away, the gas coming off at the commencement of 

 boiling was collected, and also that coming off when the liquid 

 had all but boiled away. On testing the samples for radio-activity 

 the former was found to be slightly radio-active but not nearly so 

 much so as before liquefaction, while the second was extraordinarily 

 radio-active, its activity being quite thirty times that of the 

 original gas : thus showing, as we should expect from its great 

 density, that the radio-active gas is much more easily liquefied 

 than air. 



The liquid obtained in the preceding experiment had a very 

 strong smell of coal-gas. I must again express my thanks to 

 Professor Dewar and Mr Lennox for their kindness in making 

 these experiments. 



A discharge tube was filled with strongly radio-active gas 

 obtained as above, and the spectrum was most kindly investigated 

 by Mr Newall, who photographed it and measured the lines; no 



VOL. XII. PT. III. 12 



