substances when heated on a Nernst filament. 419 
solids, but the values given cannot be taken as measuring the 
corpuscular emission from the lime-covered filament, for the 
following reason. A resistance of 5500 ohms was included in 
the circuit to prevent damage to the high potential battery, and 
while this resistance remains constant during the experiments 
the resistance offered by the discharge tube diminishes greatly 
with increasing temperature. Thus the fall of potential across 
the tube decreases as the temperature of the filament is raised, 
and as the current is not saturated, its values at the different 
temperatures are not proportional to the number of corpuscles 
emitted by the filament. The potential ditference required for 
saturating a thermionic current increases as the temperature 
rises, and at the same time the energy required to produce 
ionisation by collisions in the gas decreases. At the temperatures 
obtained with a glowing Nernst filament ionisation by collisions 
occurs under quite small electric forces—much smaller than that 
due to the heating circuit in these experiments—and even at the 
low pressure used it was not possible to obtain a saturation 
current. 
A comparison of the results in the above table with those 
recorded in the curve of fig. 1 shows that the negative ionisation 
produced by a glowing Nernst filament is enormously increased by 
coating the filament with lime. It is difficult to say whether the 
emission is as large as that from lime-covered platinum, but the 
experiments at all events show that the order of magnitude is 
the same in both cases. To make a careful comparison of the 
two it would be necessary to work at much lower temperatures 
so as to avoid the complications introduced when the discharge 
becomes luminous. It was found to be extremely difficult to 
maintain the lime-covered filament at a constant temperature 
much less than 1500°C., and after several attempts the idea of 
trying to make an exact comparison was abandoned. The results 
described above are, however, sufficient to show clearly that the 
large negative emission obtained from lime heated on platinum 
cannot be attributed to the metal with which it is in contact. 
The positive emission from Sodium Phosphate. 
A new filament was used in these experiments, and the 
positive emission from this was first investigated. It was found 
that the emission in a good vacuum decreased on continued 
heating of the filament, but the decrease was not nearly so great 
as that obtained when platinum is first heated in a vacuum. ‘The 
author has shown that the positive emission from platinum is 
largely due to absorbed gas, and in the case of a Nernst filament 
it may also be due to gas evolved during the heating, in which 
28—2 
