substances when heated on a Nernst filament. 417 
‘exponential manner usually obtained with substances heated at 
lower temperatures. Most of the current-temperature curves 
‘obtained with Nernst filaments at high temperatures showed 
‘this approximately linear relation, but the curve selected for 
fig, 1 is that in which the experimentally determined points lie 
‘most nearly on a straight line. A negative ionisation of about 
‘the magnitude represented by this curve was always obtained 
when a Nernst filament was heated in a good vacuum. The 
large potential difference of 214 volts was used to swamp the 
alternating potential difference of the heating current, which, 
between the ends of the filament, was about 60 volts when the 
filament was at 1500° C. 
In the first experiments with lime-covered filaments, the lime 
was obtained by evaporating a strong solution of calcium nitrate 
on the filament and then igniting, but this process was found to 
spoil the filament, which became very difficult to start glowing, 
and when glowing would sometimes suddenly go out during the 
observations ; a filament so treated also broke through after being 
used for a short time. Pure lime (Kahlbaum, prepared from 
marble) was therefore powdered very finely in an agate mortar, 
and some of this powder was stirred up with distilled water. 
‘The lime in suspension in water was then placed upon the 
filament, a drop at a time, and the water was evaporated away by 
Warming over the heater. The drops were all placed near the 
centre of the filament, and care was taken not to allow any of the 
liquid to go on to the platinum leads, although this precaution 
‘was probably unnecessary, for the leads never became visibly hot 
during the experiments. The evaporation of the lime water was 
continued until a uniform layer of lime was obtained over about 
7mm. of the middle of the filament. When obtained in this 
manner the layer of lime did not peel off on heating, and the 
filament could be started glowing just as easily as a new one. It 
should be mentioned that the filaments used were all of about the 
same dimensions, the length of the glowing portion being about 
9°5 mm. and the diameter ‘78 mm. 
The glowing lime-covered filament was placed in the apparatus 
and the air-pressure was reduced. It was at once obvious that 
under these conditions the lime gives an enormous negative 
emission, for as the pressure was reduced to a few millimetres 
a brilliant glow appeared in the discharge tube, without the 
application of any electromotive force except that of the heating 
circuit. One of the platinum wire leads to the filament was kept 
earthed, and from the appearance of the discharge it was seen 
that the bare end of this wire, near the filament, was acting as an 
anode. By lowering the temperature of the cathode the luminous 
discharge could be stopped, but at the lower temperature it could 
VOL, XVII. PT. Y, 28 
