128 H. 2L. Dadourian — Radio-activity of Thorium. 



insulated from the tin vessel and connected to the negative 

 terminal of a battery of 400 volts, the other terminal being- 

 connected to the tin vessel. 



After exposures of 19 hours the copper plates were taken 

 out of the charging vessels and introduced into a testing vessel 

 connected with a Dolezalek electrometer, and the activity 

 observed as usual. Another testing vessel, containing a ura- 

 nium oxide standard, was connected with the electrometer, 

 in parallel with the first testing vessel, in order to test the sen- 

 sitiveness of the electrometer during the experiments and allow 

 for changes in the sensitiveness if there were any. The elec- 

 trometer had a sensitiveness of about 200 ems per volt with 100 

 volts on the needle and the scale at a distance one meter from 

 the mirror. 



The emanation generated at a given horizontal layer of the 

 solution has to diffuse through the liquid above that layer in 

 order to reach the surface. Therefore on account of the 

 rapid decay of the thorium emanation (half-value period 

 being 51 seconds) the percentage of the emanation, generated 

 at that layer, which reaches the surface of the solution, will 

 depend upon the depth of the layer. Thus the concentration 

 being the same, the emanation which crosses a square centi- 

 meter of the surface per second depends upon the depth of 

 the solution, provided it is not so deep that the emanation 

 produced at the bottom has time enough to decay to a negligi- 

 ble value before reaching the surface, in which case it will be 

 independent of the depth of the solution. The solutions used 

 in these experiments were not deep enough to fulfill the above 

 condition, therefore it was necessary to make all the solutions 

 of the same depth. The equality of depth was realized by 

 putting equal volumes (50 oc ) of each solution in the flat- 

 bottomed dishes, which were of the same diameter (9'5 cms ). 

 For similar reasons the negatively charged plates were kept at 

 equal distances from the surface of the solution to which they 

 were exposed. 



The solutions tested were of different concentration, there- 

 fore in order to be able to compare their activities it was 

 necessary to determine the quantitative relation between the 

 concentration of the solution and the amount of emanation 

 obtained. Two solutions of thorium nitrate were prepared, 

 one of which had a concentration twice as great as the other. 

 Equal volumes of these solutions were placed in fiat-bottomed 

 glass dishes of equal diameter, thus securing equal depths. 

 Two copper plates were exposed to these solutions under 

 similar experimental conditions and introduced into the test- 

 ing vessel. The ionizations produced by the active deposits 

 obtained from the more concentrated and the less concentrated 



