600 Dr. B. B. Boltwood on 



to collect it and measure its activity. Most solids have, 

 however, a certain emanating power, i. e.,' they lose at 

 ordinary temperatures a certain proportion of their emanation. 

 In the case of thorium and radium compounds this phenomenon 

 has been examined by Rutherford *, who has observed that 

 the emanating power of the compounds is subject to great 

 variations under different physical conditions. Rutherford 

 found that the emanating power of ordinary thoria is per- 

 manently reduced about 90 per cent, when this substance is 

 heated to a white heat, while the effect of high temperature 

 on compounds of radium is likewise to decrease their power 

 of giving off emanation at ordinary temperatures. 



It will be shown in the course of this paper that marked 

 differences in emanating power are exhibited by different 

 mineral species and by different varieties of the same mineral 

 substance. In determining the equilibrium quantity of 

 emanation from these minerals it is therefore necessary to 

 consider this factor, and either to determine it separately or 

 else to compensate for it. In the methods described in 

 previous papers, the compensation was accomplished by 

 dissolving the mineral in a closed vessel and allowing the 

 solution to stand until equilibrium had been reached. This 

 plan, however, involved practical difficulties in the cases of 

 many mineral substances, and has been abandoned in favour 

 of the more direct method of separately determining the 

 emanating power of each sample of material used in the 

 investigation. 



The measurement of the emanating power of the various 

 substances was conducted as follows : — A sample of any 

 given mineral having been selected, it was first crushed and 

 then very finely pulverized in an agate mortar. A portion 

 of this powder, weighing from 0*5 to 1*5 gram, was carefully 

 weighed out and introduced, between short plugs of cotton- 

 wool, into a glass tube about 15 cm. in length and 7 mm. 

 in diameter, the ends of which were drawn out into short 

 capillary tubes. A slow current of air was then drawn 

 through the tube in order to remove any emanation clinging- 

 to the solid, and the tips of the capillary tubes were sealed 

 in the flame of the blowpipe. The tube was allowed to stand 

 for forty days, and the emanation which it contained was 

 removed by drawing through it about 100 c.c. of air, which 

 was collected in a burette, allowed to stand for about fifteen 

 minutes, and then introduced into an electroscope. The 

 observed activity of the emanation divided by the weight of the 

 material taken gave the activity of the emanation lost by one- 

 * Phys. Zeit. ii. p. 429 (1901). 



