31 209 



natural logarithms, p the weight (in grams) of the uianinite employed, and / the 

 lime in seconds reckoned from the moment the solution was sealed up ; p and ' 

 are found by experiment. 



From the last equation we get q in the above proposed unit (per gram uranium 

 per second), wliile a direct measurement of (he emanation in the ionisation chamber 

 gives the emanation in the arbitrary unit by means of a,„. The factor ;-, which 

 a,,, is to be multiplied by to express the emanation in the right terms, is given by 

 the equation, 



q — r«;n- 



As a mean of two experiments I got 



r = 1.347. 



According to measui-ements made by Rutherford and Boltwood', natural minerals 



contain 3.8 x 10~' gr. of radium per gram of uranium. Therefore the proposed 



unit of emanation is produced by 3.8 x 10~' gram of radium per second. Thus 



we are enabled to refer this unit to Curie and Laborde's - unit of emanation. 



In the proceeding exposition I have taken it for granted that the relation 



between the emanation contained in the ionisation chamber and the ionisation factor 



a,„ is constant. But this only holds good when the testings are carried out at 



even density of the atmosphere. A change in the density of the atmosphere causes 



a corresponding change in the relation between the emanation and the ionisation 



produced. If alterations in the ionisation called forth by small variations in the 



atmospheric density are assumed to be proportionate to the alterations of the 



densitv, I have 



£-£' d — d' 



where E and fî' represent the ionisation produced by the same emanation, i. c. when 

 the density of the air is d and d' respectively, and fj. a factor of proportionality. 

 Setting aside the moisture of the air, 1 refer all the measurements to the pressure 

 of 760 mm. mercury, and 18° C, so that d = 0.001213. 



By direct measurements of a given quantity of emanation, I determined the 

 leakage due to the emanation, at 766 mm. pressure and 20" C, to 889 arbitrary units. 

 The same emanation caused the leakage to be 852, at a pressure of 618 mm. and 

 20° C. The density of the air at 766 mm. and 20° is d = 0.001215, and at 618 mm. 

 and 20° C, d' ^^ 0.000980. Then we have from the experiments mentioned, 



0.001215 -0.000980 _ 889 — 852 

 ^ ÔT001215 ~" 889^ 



or n = 0.215. From the formula 



E 

 E 



' Amer. Journ. Sei. 22, 1, 1906.. 

 - Comp. rend. 138, 1180, 1904. 



E-E' d — d' 



