SECTION ITI., 1912. [11] Trans. R. S. C 
On the Diffusion of Actinium Emanation and the Active Deposit Produced 
by at. 
By Proressor J. C. MCLENNAN, 
(Read May 16, 1912). 
In a paper by W. T. Kennedy, published in the Phil. Mag., of 
November, 1909, an account is given of some experiments on the active 
deposit products of actinium. 
In one series of experiments, observations were made on the dis- 
tribution of the active deposit obtained on two parallel plates, 2 mm. 
apart, and charged to 250 volts, the actinium salt being so placed 
immediately below the plates that the emanation from it was compelled 
to diffuse up between them. In these experiments the plates and the 
salt were inserted in a vessel in which the air could be maintained at 
any selected pressure and the observations were made on the distribu- 
tion of the active deposit obtained on the plates with air pressures of 
760 mm., 120 mm., 25 mm., and 5 mm., respectively. The distributions 
of the deposit obtained at atmospheric pressure on the negatively, 
charged plate, and also on the positively charged one, are shown in Fig. 
1. From the curves it will be seen that on both the positively charged 
plate and on the negative one the active deposit was greatest close 
to the salt, and was distributed in gradually decreasing amounts at 
points on the plates farther and farther from the salt. The curves 
given in the paper for the distributions corresponding to the other 
pressures were of the same general character. It was found, however, 
that while at atmospheric pressure by far the greater part of the active 
deposit was found on the negatively charged plate, at the other pres- 
sures investigated, the deposit on the positively charged plate became 
relatively greater, and at 5 mm. was almost identical in amount with 
that obtained on the negative plate. 
In the second series of experiments, observations were made on 
the amount of deposit obtained on the plates at a point 2.5 em. above 
the salt when the pressure of the air in the vessel in which plates and 
salt were placed was gradually diminished. The results for this set 
of observations are given in Table I, and the curves representing them 
are shown in Fig. 2. 
