578 Mr Gill, A Preliminary Note on an Effect observed 
A Preliminary Note on an Effect observed when Palladium Foil 
is heated in Air at a low Pressure. By Rev. H. V. Gill, Downing 
College. (Communicated by Professor Thomson, F.R.S.) 
\Reacl 18 May 1908,] 
When making some preliminary experiments in connection 
with an investigation, suggested by Professor J. J. Thomson, con- 
cerning the discharge of electricity from palladium electrodes a 
phenomenon was observed which does not seem to have been 
hitherto noticed. The object of this note is onty to put on record 
the fact as observed, reserving a fuller account of experiments 
which are being made at the Cavendish Laboratory to determine 
the nature and causes of the effect. 
A vacuum tube had been fitted up containing a piece of 
palladium foil about an inch long and ^ inch wide, mounted at the 
extremities of two leads so that its temperature could be raised by 
means of an electric current. This tube was sealed on to a mercury 
air-pump, pressure-gauge, and drying bulb, and could be separated 
from them by means of a mercury seal. 
The foil was gradually heated in air at a pressure of about 
0T5 mm. When the palladium was at a white heat there became 
visible around it a glow, not unlike the “negative glow” in a 
vacuum tube discharge. There was, however, no electric field 
except that due to the current from the two storage cells em- 
ployed to heat the foil. 
The glow is of a rich purple-blue colour, and is quite visible in 
a darkened room in spite of the glare from the white-hot palladium. 
On the first occasion this glow was observed in a tube about one 
inch in diameter, and appeared to extend to the walls of the tube. 
Afterwards bulbs about four inches in diameter were made use 
of (see fig.). There appeared to be a “ dark space ” between the 
hot metal and the glow, but it is not easy to decide if there really 
is a space. What appears to be an interval between them may be 
merely the effect of contrast due to the glare. However, the 
majority of those who have seen this phenomenon are inclined 
to think there really is a “dark space.” In bulbs of the size just 
mentioned the glow does not extend to the walls, but formed 
