21c Coy and Ashman — Urano-Uranic Oxide. 



525 



sample C may be due to a trace of impurity ; but the value is 

 sufficiently close to the mean for A and B to show that the 

 method is satisfactory. 



We next made a new set of films on flat copper plates which 

 were cut on a lathe and were almost perfectly circular. These 

 plates differed slightly in size, however, for which reason 

 the diameter of each plate was measured (in two or more 

 directions) with a comparator, capable of giving results accu- 

 rate to 0*00 l cm . The IT 3 8 for these films was prepared in a 

 single portion from sample A. On account of the large 

 quantity of material used it was necessary to continue the 

 heating at 700° for about three hours in order to obtain con- 

 stant weight. The activities are shown in Table III ; the last 

 column gives the corrected activity for exactly 7 cm diameter. 









Table III. 











a + 8 









Corrected 



No. 



Weight 



Activity 



8 Activity 



a Activity 



Diameter 



a Activity 



30 



0-6812 



1-0800 



0263 



1-0537- 



7-0168 



1-0487 



28a.,. 



0-8776 



1-0788 



0-0347 



1-0441 



6-9912 



1-0467 



29a. __ 



0-7996 



1-0861 



0-0300 



1-0561 



7-0174 



1-0509 



296. _. 



0-7770 



1-0812 



0-0288 



1-0524 



70174 



1-0472 



31a___ 



0-7801 



1-0935 



0-0326 



1-0609 



7 0301 



1-0519 



33 



0-7070 



1-0737 



.0-0270 



1-0467 



6-9969 



1-0476 



34 



0-7677 



1-0796 



0-0321 



1-0475 



69948 



1-0491 



34«.._ 



0-9107 



1-0891 



0-0364 



1-0527 



6-9948 



1-0543 



Mean. 



]-0495 



The activities in Table III are also expressed in terms of an 

 arbitrary unit, about three per cent greater than the unit used 

 for Table II. One of us (loc. cit.) has proposed to take as the 

 unit of radio-activity, the activity due to one sq. cm. of such 

 films of U 3 O s as those to which table III refers. In terms of 

 such a unit, the activity of each film of Table III is represented 

 by its area. 



Several years ago, Rutherford and McClung* determined 

 the ionization current of layers of U 3 8 of different thickness, 

 when placed between parallel plates sufficiently far apart for 

 the complete absorption of all the tf-rays in the air between 

 the plates. The saturation current per sq. cm. of surface was 

 4-0 X 10- 13 ampere, for a weight of 0-0189 g. of U 3 8 per sq. cm. 

 This weight is great enough to give the maximum a-ray activ- 

 ity provided the material is in the form of a perfectly uniform 

 film ;. but this was not the case in Kutherford and McClung's 

 experiment, as the oxide was merely "dusted" on the plate 

 and therefore the observed current was far below that for a 

 ♦Phil. Trans. A., cxcvi, 52, 1901. 



