Emanation and Active Deposit produced by it. 373 



From these carves it will be seen that a maximum deposit 

 was obtained on the negative plate at the point under obser- 

 vation at a pressure o£ approximately 80 mm., while the 

 maximum deposit on the positive plate was not obtained 

 until a pressure of 16 mm. was reached. 



When these experiments were repeated with carbon dioxide 

 and with hydrogen in place of air it was found that the 

 maximum deposit at the same point on the negative plate 

 was obtained with a pressure of 60 mm, in carbon dioxide, 

 and with a pressure of 250 mm. in hydrogen. 



Further, in some additional observations made by Kennedy, 

 since the publication of the paper referred to above, it wfts 

 found on measuring the deposit on tlje negative plate at 

 different points at various distances from the salt that q, 

 simple law existed connecting the pressure corresponding to 

 the maximum deposit obtained at a given observation poin£ 

 with the distance of that point from the salt. Recording tq 

 this law, if ay, represents tfye distance from the salt of tl^e 

 point at which the deposit was investigated, and p x tbe 

 pressure which gave a maximum deposit at that point, then 

 pi was found to vary inversely with #,, or 



p 1 £j,==a constant *, 



In seeking an interpretation of these results it seems clear 

 from the first set of observations that the maximum deposits 

 found in the second series of observations were not due to 

 the presence in the space between the plates of a maximum 

 concentration of either the emanation from the salt, or of the 

 active deposit particles produced by the emanation, but that 

 they arose from the faGt that the emanation from actinium 

 was an exceedingly short-lived radioactive product. On 

 account of this characteristic the emanation at higher pres- 

 sures, snch as atmospheric, was wholly transformed before it 

 diffused very far from the salt, and but little of it therefore 

 reached the point on the plates under observation in the exr 

 periments illustrated by the curve in tig. 2, With decreasipg 

 pressures, however, the emanation diffused more and more 

 rapidly, and the effect of this produced at first an increase in 

 the concentration of the emanation at the point of observa- 

 tion. With the pressures decreased still further the emana- 

 tion diffused more and more rapidly, and finally at the lpvvest 

 pressure obtainable the diffusion became so rapid that the 

 concentration at the point of observation became exceedingly 

 small. With this explanation it follows that the amount of 



* Trans. Brit. Assoc. 1910, p. 543. 

 Phil. Mag. S. 6. Vol. 21. No. 141. Sept 1912. 2 C 



