Perkins — Molecular Weight of Radium Emanation. 467 



be clone by having the electroscope filled with hydrogen. 

 This would be inconvenient and involve considerable liability 

 to error due to differences in ionizing power produced by the 

 presence of any small amount of air. A statement made by 

 Rutherford and Soddy* seemed, however, to give an easy 

 solution of the difficulty. They say : 



" If either emanation is conveyed by a slow stream of hydro- 

 gen, oxygen or air through a metal spiral immersed in liquid 

 air, no trace of emanation escapes in the issuing gas." 



Provided this were true, after diffusion the emanation could 

 be condensed by means of liquid air, the hydrogen replaced 

 by air and, as soon as room temperature was reached, the 

 emanation mixed with air could be introduced into the elec- 

 troscope. In order to test this statement, a long spiral of cop- 

 per tubing was made just large enough to fit inside a Dewar 

 bulb filled with liquid air. A very slow current of air mixed 

 with emanation was passed through the spiral, when it was 

 found that enough emanation remained uhcondensed to give a 

 decided leak in an electroscope placed at the issuing end, thus 

 forbidding the adoption of such a method. 



A series of experiments showed that very good results were 

 given for the relative amounts of diffused emanation by 

 measuring the amount of the active deposit (Radium C) 

 induced on the negative electrode. Using this method of 

 measurement, the order of experiment was as follows : 



Measure the natural leak of the electroscope while bringing 

 the furnace into equilibrium at the required temperature. 

 Introduce into the diffusion chamber, by displacement, 

 emanation boiled from 4 grs. of carnotite dissolved in hydro- 

 chloric acid, the containing flask having been previously filled 

 with hydrogen. This is an amount of emanation equivalent 

 to that in equilibrium with 9*2 XlO -8 grs. of radium. Let dif- 

 fusion take place for 10 min. into the open air, so as to allow a 

 sufficient time for the emanation to get well mixed with the 

 hydrogen in the diffusion chamber. Collect the first sample 

 for 20 min. in one bottle; then transfer, by means of a " T " 

 tube and pinchcocks, into the other bottle for a second period 

 of 20 min. Exhaust the electroscope and introduce sample 1, 

 leaving sample 2 under a negative pressure so that any leak will 

 be into the collecting bottle rather than out into the air. After 

 3 hrs., draw a rapid current of air through the electroscope by 

 means of a water pump to displace the emanation, and make 

 a series of measurements of the ionization produced by the 

 active deposit. Introduce sample 2 and after 3 hrs. measure 

 the activity of the active deposit as before. Correct the values 

 obtained for sample 2 for the decay of the emanation for the 

 time which has elapsed between the introduction of sample 1 



*Phil. Mag., May, 1903. 



