McCirettanp— On Secondary Radiation and Atomie Structure. 3 
case reversed the proper order. Possibly the substances of very small density 
may have too small a value assigned to them, as in their case a thick plate or layer 
was necessary, which could not, therefore, be in quite the same position relative 
to the ionisation-tube 7. In every case the upper surface of the substance was 
7 ELecTROMETER 
WE Ile 
placed in exactly the same position. Of course corrections were always applied _ 
for whatever small ionisation existed when the plate P was removed, and the 
stand S lowered. 
