Diffusion of Thorium and Actinium Emanations. 639 



to 20 hours, and no electric field was applied. Four or five 

 hours after the removal of the plate, when the active deposit 

 was decaying according to an exponential law, and correc- 

 tions could be more conveniently applied, the activities of 

 the strips in pairs, one from each side at the same distance 

 from the bottom, were measured in a sensitive a-ray electro- 

 scope. 



Within the limits of experimental error the emanation 

 ought to be distributed according to an exponential law * 



/X 

 p=Po e ~ v d x , where p is the initial concentration, p the con- 

 centration at a point at distance x, X the radioactive constant, 

 and D the coefficient of diffusion. If the active deposit reaches 

 the plate at the level at which it is formed, the variation in 

 the activity of the strips should follow the same law. This 

 was not the case, however. The logarithm of the activities 

 plotted against the distances x did not give a straight line 

 but a curve, according to which the coefficient of diffusion 

 apparently increased with the distance from the source. 



Two parallel plates, 2 or 3 mm. apart, without a field, as 

 in Debierne's and Bruhat's experiments with actinium 

 emanation |, were then substituted, the distribution of 

 emanation between them being examined; but still depar- 

 tures from an exponential law were observed. It was 

 thought probable that this might be caused by the diffusion 

 of some of the active deposit so that it reached the plates at 

 a level other than that at which it was formed from the 

 emanation. According to some observations made by Mme. 

 Curie on radium deposit J, if the air containing emanation 

 is not perfectly dry, agglomerations of molecules of water 

 vapour and active deposit are probably formed. As the 

 phenomenon observed might have been due to these, a tube 

 of phosphorus pentoxide was placed in the cylinder to dry 

 the air, but apparently a certain amount of disturbance still 

 took place. 



Next an electric field was employed § to direct the active 

 deposit more rapidly to the plates. With 2-4 volts, and 

 even with 40 volts, some trace of the same phenomenon 

 remained; but with 80 volts practically a straight line was 

 obtained when the logarithms of the activities were plotted 

 against distance. In all subsequent experiments a difference 



* Rutherford, ' Radioactivity/ p. 276. 



t Loc. cit. 



X Mme. Curie, Comptes Rendus, 1907, pp. 477 & 1145. 



§ Rutherford, loc. cit. ; Russ, loc. cit. 



