Substance emitted from Thorium Compounds. ?> 



In the above table Ihe rate of leak with the thorium oxide 

 uncovered is taken as unity. It will be observed that the 

 first layer reduced the rate of leak to '74, and the five suc- 

 ceeding layers produce very little effect. 



The action, however, is quite different if we use a thin * layer 

 of thorium oxide. With one layer of paper, the rate of dis- 

 charge is then reduced to less than -^ of its value. At first 

 sight it appears as if the thorium oxide gave out two types of 

 radiation, one of which is readily absorbed by paper, and the 

 other to only a slight extent. If we examine the radiation 

 given out by a thin layer of thorium oxide, by placing suc- 

 cessive layers of thin paper upon it, we find the radiation is 

 approximately homogeneous, as the following table shows. 



Table II. 

 Thickness of Paper = *0027 cm. 



[Number of Layers of 

 Thin Paper. 



Rate of Discharge. 



i 





 i 



2 



3 



1 



1 



•37 

 •!« 



•08 



The rate of leak of the bare salt is taken as unity. If the 

 radiation is of one kind, we should expect the rate of discharge 

 (which is proportional to the intensity of the radiation) to 

 diminish in geometrical progression with the addition of equal 

 thicknesses of paper. The above figures show that this is 

 approximately the case. With a thick layer of thorium oxide, 

 by adding successive layers of thin paper, we find the rate of 

 discharge gradually diminish, till after a few layers it reaches 

 a constant value. The amount that is cut off by the first 

 layer of foolscap-paper (see Table I.) is of the same kind of 

 radiation as that which is emitted by a thin layer of oxide. 



On directing a slight current of air betw een the test-plates, 

 the rate of discharge due to a thick layer of thorium oxide is 



* To produce a thin layer on a plate, the oxide, in the form of a fine 

 powder, was sprinkled by means of a tine gauze, so as to cover the plate 

 to a very small depth. By a thick layer is meant a layer of oxide over a 

 millimetre in thickness. 



B 2 



