in the Discharge through Gases at Low Pressures. 147 
discharge struck even the smallest particle of mercury the 
cathodes became amalgamated *, a phenomenon which was 
most marked in the case of gold. 
In order to confine the disintegration to the front of the 
cathode, the back was covered by a disk of mica and the sup- 
porting wire was surrounded by a narrow glass tube, 
et, 
A few millimetres below the cathode a platinum wire S was 
sealed into the tube, and the difference of potential between 
this and the cathode was measured with Braun electrometers, 
one reading to 1500 V. and the other to 10,000 V. During 
each series of observations, the difference of potential was kept 
as constant as possible by regulating the gas-pressure. As this 
was found to be a rather difficult matter, a glass tube G of 
1 litre capacity was connected with the apparatus in order to 
diminish the lowering of the gas-pressure, and the accom- 
panying increase of potential-difference which took place 
when the current flowed. Notwithstanding this, it was 
frequently necessary to admit air from the Kahlbaum pump 
through the cock H, into the apparatus. With a little practice 
it became possible, by turning the cock more or less rapidly, 
to introduce the desired quantities of gas, if the pressure in 
the pump was kept a little higher than in the discharge 
apparatus. The pressure was measured in the ordinary way, 
by means of a MacLeod manometer M. | 
A 30-plate Toepler machine furnished the current, which in 
most of the experiments was kept constant at 0°6 x 10~* amp. 
It was found possible to prevent the current from varying 
‘'* The amalgamation of Al cathodes was observed by E. Warburg 
(Wied. Ann. xxxi. p. 577, 1887). 
