Condensation of the Radioactive Emanations. 575 



temperature o£ liquid air. The method employed was to pass 

 a slow steady stream of the thorium emanation, mixed with 

 hydrogen or oxygen, into the copper spiral immersed in 

 liquid air for a space of five minutes. The emanation was 

 thus condensed in the spiral. The current of gas was then 

 stopped, and after definite intervals, extending in successive 

 experiments from one to five minutes, the spiral was rapidly 

 removed from the liquid air, plunged into hot water, and the 

 emanation present swept rapidly with a current of air into a 

 large testing-vessel. The results showed that the emanation 

 lost its activity at the same rate at the temperature of liquid 

 air as at ordinary temperature, i.e. its activity fell to halt 

 value in about one minute. This is in agreement with results 

 previously noted for other active products, showing that the 

 rate of decay is unaffected by any physical or chemical 

 agency. 



Summary of Results. 

 The results show that the thorium emanation begins to 

 condense at about — 120° C. The rapid rate of decay of its 

 activity renders a determination of the point at which the 

 condensed emanation commences to volatilize experimentally 

 impracticable. But under all conditions tried some of the 

 emanation escapes condensation at temperatures much below 

 the temperature of initial condensation. In a slow stream 

 of gas the presence of the emanation is first observed at about 

 — 155° C. It is probable that —120° represents the true 

 temperature of volatilization and condensation, and that the 

 escape of emanation below this temperature is due to the 

 extremely small number of condensing particles present. 

 The radium emanation commences to volatilize at —153° in 

 in a steady stream of gas, and at —150° in a stationary 

 atmosphere, and this latter value may be accepted with con- 

 siderable confidence as being the true temperature of initial 

 \olatilization. In the case of radium there is no sensible 

 difference between the temperature of volatilization and of 

 condensation, and the whole of the emanation is condensed at 

 temperatures only slightly below the initial point of volatiliza- 

 tion. This difference of behaviour of the two emanations is 

 explained on the view that the number of particles of emana- 

 tion present for equal effects is probably many thousand times 

 greater in the case of radium emanation than in the case of 

 thorium emanation. All the radium emanation is volatilized 

 within a very few degrees of the initial point, the rare of 

 volatilization of course depending on the rate of rise of 

 temperature. But with a very slowlv vising temperature. 



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