G. C. Ashman — Determination of Radium Emanation. 121 



should yield identical results. Such duplicate determinations 

 were carried out by dividing the air current which had passed 

 through the purification train and passing each half through a 

 separate coil. In this way equal volumes of air passed through 

 the coils simultaneous!} 7 under exactly similar conditions. 

 When corrections for the lapse of time between the observa- 

 tions on the contents of the coils were made, it was found that 

 each had collected emanation corresponding to 131 XlO -12 

 gram of radium. This furnished additional proof that the 

 method can be relied upon for quantitative results. As a 

 mean of all my experiments the amount of radium necessary 

 to produce the observed amount of emanation per cubic meter 

 of air is 96x10 12 , or nearly 10~ 10 gram. This is about 25 per 

 cent higher than the mean value found by Eve by the char- 

 coal absorption method. The difference may easily be due to 

 the variations in the quantity measured. Neither method is, 

 however, entirely above criticism. It seems that aside from 

 the rather lengthy and complicated nature of Eve's experi- 

 ments there are two possible sources of error. One is that 

 Eve assumes in his calculations that increasing the amount of 

 emanation in the air three- or fourfold by the introduction of 

 a standard radium solution does not affect the fraction of the 

 emanation absorbed by the charcoal. This has not been tested 

 experimentally. The other is the fact, not taken into account, 

 that the emanation is not completely removed from a radium 

 solution by the bubbling method. The application of this 

 correction, however, would reduce Eve's results to a still lower 

 value. Perhaps some of the difference between our results 

 may be accounted for by the fact that in Eve's experiments 

 the air was taken at a level of the fourth floor of a building, 

 while in the experiments described here the air was taken at 

 the earth's level. In my own experiments the fact that the 

 second coil, after the passage of the air through the first, did 

 not contain an amount of emanation detectable by our electro- 

 scope only proves that under the conditions of the experiment 

 the first coil collected all the emanation capable of condensa- 

 tion at the temperature of liquid air. The complete conden- 

 sation is limited only by the value of the vapor pressure of 

 radium emanation at the temperature of liquid air. The 

 experiment of Rutherford and Socldy* indicates that the 

 vapor pressure below the condensation point is practically 

 zero, while Ramsayf observed luminous bubbles passing down 

 the walls of the vacuum tube while exhausting a vessel con- 

 taining the frozen emanation. This would indicate an appre- 

 ciable vapor pressure even at the temperature of liquid air. 

 The net results of all experiments show that the amount of 

 *Loc. cit. f J. Chem. Soc, xci, 932, 1907. 



