640 Miss M. S. Leslie : Comparison of the Coefficients of 



of potential o£ 160 volts was used. The measurements were 

 begun about 1*5 cm. from the end of the cathode plate, and 

 the strips of tinfoil did not extend the whole width of the 

 plate, but were several millimetres shorter ; so that any effects 

 due to non-uniformity of the field at the edge of the plate 

 might be avoided. Measurements were made at atmospheric 

 pressure, and also at reduced pressures. The product of the 

 pressure P and the coefficient of diffusion D should be a 

 constant at constant temperature, varying as the square of 

 the absolute temperature. The values of P x D given below 

 have been corrected for 15° C. The coefficient of diffusion 

 was calculated assuming 54 seconds * as the period of trans- 



P. 



PXD (corrected). 



76-2 cm. 

 76-7 „ 



HI } at 80 V0 



75-8 „ 



6-07 



75-65 „ 



6-69 



755 „ 



6-00 



73-8 „ 



6-43 



54-95 „ 



6-74 



37-2 „ 



6-80 



31-2 „ 



6-89 



29-7 „ 



6-44 



12-8 „ 



6-20 



8-8 „ 



6-39 





Mean . . 6*46 



The value of D at 76 cm. pressure and 15° C. = -Q85. 



The apparatus was sufficiently long to prevent the necessity 

 for additional corrections at reduced pressures. 



Experiments with Aetinium — The thorium hydroxide was 

 replaced by an actinium preparation, and the rate of diffusion 

 determined in exactly the same way. All the experiments 

 were made at reduced pressures, since at atmospheric pressure 

 only a very small quantity of emanation reached the point 

 at which the measurements were begun. An ordinary a-ray 

 electroscope was used instead of the sensitive one employed 

 in the experiments with thorium emanation. The period of 

 transformation of actinium emanation f was taken as 3' 9 sees. 

 or X-l-SxlO" 1 sec." 1 . 



# See note later. 



t See note at the end. 



