Substance emitted from Thorium Compounds. 

 Aluminium Foil, thickness = *0008 cm. 



Number of Layers. 



Rate of Discharge. 







1 

 3 



6 



i 



1 

 •66 

 •42 

 •16 



Cardboard, thickness "08 cm. 



Layers. 



Rate of Discharge. 





 1 

 2 



1 

 •40 

 •21 



The emanation passed readily through several thicknesses 

 of gold- and silver-leaf. A plate of mica, thickness '006 cm., 

 was completely impervious to the emanation. 



When a thick layer of thorium oxide, covered over with 

 several thicknesses of paper, is placed inside a closed vessel, 

 the rate of discharge due to the emanation is small at first, but 

 gradually increases, until after a few minutes a steady state 

 is reached. 



These results are to be expected, for the emanation can 

 only slowly diffuse through the paper and the surrounding 

 air. A steady state is reached when the rate of loss of intensity 

 due to the gradual decay of the radio-activity of the emanation 

 is recompensed by the number of new radio-active centres 

 supplied from the thorium compound. 



Let n=number of ions produced per second by the radio- 

 active particles between the plates. 



Let <7 = number of ions supplied per second by the emanation 

 diffusing from the thorium. 



The rate of variation of the number of ions at anv time t is 

 given by 



dn 



where \ is a constant. 



