Condensation of the Radioactive Emanations. 567 



With hydrogen streams from 25 c. c. to 1*38 c. c. per 

 second and for the slow stream of oxygen the results are in 

 good agreement and give a mean value for T 1 of —153°, for 

 T 2 — 151 0, 5. But a well-marked difference appears when 

 the stream of hydrogen is increased to 23 c. c. per second. 

 This corresponds to an initial velocity of 50 cms. a second 

 "through the spiral, and 20 cms. per second after the temperature 

 of the bath has been obtained. The result is therefore to be 

 expected, for in such a rapid stream the gas is not cooled 

 down to the temperature of the spiral and the volatilization- 

 point of the emanation is in consequence apparently lower. 

 For the same reason the temperature observed for the oxygen 

 stream of only *58 c. c. a second is probably too low, for this 

 gas is cooled with more difficulty than hydrogen. Even if 

 the temperature of the spiral is ultimately attained, a rapid 

 current of gas would tend to sweep out the emanation it had 

 volatilized in its passage, without giving it time to be 

 recondensed in the subsequent portions of the spiral. This 

 -effect, for reasons to be discussed later, would also be greater 

 in oxygen than in hydrogen. Further determinations for 

 the radium emanation by another distinct method are given 

 later in the paper. 



At this stage some experiments may be mentioned that 

 were performed with a much larger quantity of radium 

 emanation to determine the amount that is volatilized at 

 various temperatures. In one experiment at —154°, no 

 escape of emanation was observed although less than 

 1/10000 part could have been detected. At —152° about 

 one half per cent, and at —150° considerably more than half 

 of the total amount had come off. There is no doubt that in 

 a bath, kept constant at the temperature of initial volati- 

 lization, all would volatilize if sufficient time were allowed, 

 but it is probable that the time required would be considerable. 

 There is in fact evidence that the condensed radium emanation 

 possesses what corresponds to a vapour-pressure in an ordinary 

 substance. 



Experiments for the Thorium Emanation by the same Method, 



The rapid loss of the activity of the thorium emanation, 

 which decays to half value in one minute, makes the deter- 

 mination of its volatilization-point a more difficult task. In 

 the first place too slow a gas-stream cannot be employed or 

 the ionization effects are too small. In the second place the 

 thorium compound must be retained all the time in the gas- 

 stream, unless the temperature of the spiral is made to rise 

 so rapidly that its determination becomes impracticable. The 

 results therefore bear a different interpretation from those 



