DISCHARGE FROM METALLIC SURFACES IN DIFFERENT GASES. 
447 
The photo-electric currents from the platinum electrode were found to be from 
15 to 20 times less than those from a zinc electrode under the same conditions in air 
at a pressure of 25 millims, 
The results given in Table III. are plotted in fig. G, the ordinates, as before, 
Fig. 6. 
representing the currents, and the abscissae the potentials to which the platinum is 
charged. 
ComjKirtson of Fhoto-Electric Currents in Different Gases. 
Tlie results of a number of series of observations on the plioto-electric currents 
from an illuminated zinc surface in the three gases, air, liydrogen, and carbon dioxide 
are given in the following Tables IV. to VII. The hydrogen was prepared in a 
Kipp’s apparatus from pure zinc and liydrochloric acid, and was dried l)y keeping it 
for some time in a long tube filled with lumps of calcium cldoride before permitting 
it to enter the leak apparatus. The hydrochloric acid was removed by passing the 
gas through a tube containing solid caustic potash in most of the experiments at low 
pressures; but this was found to be an unnecessary precaution, the hydrogen gas 
being given off so very slowly that no a]iprecial)le amount of the acid was carried 
over with it. The carbon dioxide was prepai'ed from marlde and hydrochloric acid, 
and was similarly dried. Before allowing a fresh gas to enter the leak apparatus, the 
latter was, exhausted until the pressure of the gas remaining in it was less than 
millim. mercury. Tlie new gas was tlien admitted, the stop-cock closed, and the 
apparatus again exhausted and refilled, so that no trace of the original gas might 
remain. Other details are given as Notes at the foot of each table. 
