•244 
mi. IIAliOLD A. WILSON ON THE 
Professor A. Smithells, Dr. H. M. Dawsox, and the writer (‘ Phil. Trans.,’ A, 
vol. 193, pp. 89-128). Using as electrodes two concentric cylinders of platinuni, it 
was found that the cuiTent was much greater when the outer cylinder was made the 
kathode than when it was made the anode. Also when the electrodes were simply 
connected to the galvanometer a current was observed from the inner to the outer 
cylinder tlirough the tlanie. Phenomena of this kind have been kno’wm for a long 
time to occur when metal electrodes are immersed in a flame. They are evidently 
closely analogous to the effects described by Elster and Geitel, referred to above. 
The discharge of electricity from platinum immersed in a Bunsen flame was further 
investigated liy the writer (‘ Phil. Trans.,’ A, vol. 192, pp. 499-528). In this paper 
it is shown that the ions concerned in the discharge between hot jdatinum 
electrodes immersed in a flame are nearly all produced at the suiface of the platinum, 
and that the negative ions have a much greater velocity, due to an electric field, than 
tlie positive ions. Consec|uently, nearly all the current is carried by negatix'e ions 
u-hich start at the negative electrode. These results enable the polar properties of 
the conductivity of tlanies to lie very simply explained. Hot platinum in a flame 
consecpiently discharges negative electricity very much more readily than positive 
electricity. 
Ill the ‘Phil. Mag.’ for December, 1899, J. J. Thomson gave an account of some 
experiments on the ratio of the charge (e) to the mass {rii) of the negative ions 
emitted liy a liot carbon filament in a vacuum. He found that it is impossible to 
olitaiii consistent results unless precautions are taken to ensure that the filament is 
ill an atmosphere of pure hydrogen. The cause of this is the gas evolved by the 
filament when it is heated. He found that by heating the hlament for a long time, 
and repeatedly pumping out the apparatus and letting in pure hydrogen, regular 
results could he obtained. When this had been done, the filament only lost negative 
electricity, whei'eas before it lost sometimes only positive and sometimes only negati\e, 
and sometimes both. He found that c/di was etjual to about 10', which is iieaily 
the same as e/'m for kathode rays, so that it is clear that the filament emits 
negatively charged coi’puscles or elections. Ihe importance of taking such precautions 
as those described by Professor Thomson, in experiments on tlie leak from hot wires 
at loM' pressures, is very gi-eat. Unless this is done, the gases evolved by the wire 
fill the apparatus, and the leak oljserved is very irregular in its behaviour. It is 
proliahle that the gas evolved by platinuni wires is mainly composed of hydrogen, 
which, as muII he shovui below, has a very large effect on the leak, even when present 
in very small quantities. 
A fiirthei’ investigation of the leak li’om hot platinum was piiblisheil liy the writer 
in 1901 (‘Phil. Trans.,’ A, vol. 197, pp. 415-441). In this paper it is shown that 
hot platinum in air at atmospheric })ressure in general discharges positive more 
readily than negative electricity, and that the rate of discharge at constant tempera¬ 
ture falls off gradually with the time and ultimately becomes very small compared 
