1904 - 5 .] Opacity of Aluminium Foil to Ions from a Flame , 907 
was due to a heat effect, and to change of surface electrification 
on the sulphur which formed the floor of M. Le Eon’s vessel, 
and it may be that his result in the experiment described 
above, in which he used the same apparatus, is due to a similar 
effect, as the leak of his electroscope decreased as the gas was 
cooled. 
These experiments were carried out at the Cavendish Labora- 
tory, Cambridge, during the winter 1904-5; and I beg to tender 
my sincerest thanks to Professor J. J. Thomson for his valuable 
advice and criticism. 
Cavendish Laboratory, 
Cambridge, June 1905 . 
{Issued separately September 30 , 1905 .) 
