Dijusion of Thorium and Actinium Emanations. 645 



value was found for thorium emanation when only a very 

 small amount of emanation was used, and when the plates 

 were thin, i. e. metal plates *5 mm. thick instead of the glass 

 plates 2 mm. thick employed later. What is the exact effect 

 produced by the thickness of the plate it would be difficult 

 to say, but it seems reasonable to suppose that a thin plate 

 in the diffusion vessel would produce less disturbance than a 

 thick one. The value obtained under these circumstances 

 was *085. Therefore, one may say that at 76 cm. pressure 

 and 15° C. the coefficient of diffusion of thorium emanation 

 is not greater than, and may possibly be less than '085. 

 Similarly the upper limit for the coefficient of diffusion of 

 actinium emanation is *098. 



Summary. 



(1) The values found for the diffusion coefficients of the 

 radioactive gases depend on the amount of ionization in the 

 diffusion vessel, and on the arrangements made for collecting 

 the active deposit. 



(2) A comparison of the diffusion coefficients of thorium 

 and actinium emanations under conditions as nearly iden- 

 tical as the differences of period of the different substances 

 concerned will allow, indicates that the molecular weights of 

 the two gases are probably very nearly equal. 



(3) The real coefficients of diffusion of thorium and 

 actinium emanations are not greater than, and probably less, 

 than '085 and *098 respectively. 



Note on tlie Periods of Transformation of Actinium 

 and Thorium Emanations. 



As a knowledge of the radioactive constants of the emana- 

 tion is necessary for the calculation of the above results, it 

 was thought desirable to employ values obtained by exactly 

 similar methods. The decay of the emanations was followed 

 directly by means of an electroscope. 



In the first attempts made an emanation electroscope was 

 partially evacuated, the leaf charged too much to be seen on 

 the scale, and air containing a small amount of actinium 

 emanation — not sufficient to drive the leaf entirely off tne 

 scale — was allowed to enter abruptly. After two or three 

 seconds, when the leaf had appeared in the microscope, scale- 

 readings were taken. The intervals of time were measured 

 by means of a swinging pendulum placed near the electio- 

 scope, so that it could be watched at the same time as 

 the scale was read. When the emanation had disappeared a 



Phil. Mag. 8. G. Vol. 24. No. 142. Oct. 1912. 2 U 



