416 
Proceedings of the Royal Society 
Lead, cadmium, zinc — 
Gutta percha, l^’o. 2. 
Iron, tin — 
Gutta percha, No. 3. 
Copper, bismuth — 
Gutta percha, No. 4. 
Antimony — 
Vulcanised caoutchouc. 
Silver — 
Gutta percha. No. 5. 
Platinum — 
Gutta percha. No. 6. 
Mercury, gold, palladium. 
The upper end of the scale is the positive, and the lower end the 
negative. 
This arrangement of the metals differs from that which results 
from my experiments chiefly in the position of tin, bismuth, and 
antimony. The positions of the latter two are contradictory to 
Becquerel’s results. 
In the experiments which I have made I have aimed at getting 
quantitative results. With an electroscope to scrutinise the electri- 
city produced, only qualitative results could be looked for ; but 
with an electrometer to scrutinise the electricity, more definite in- 
formation is possible. I had the advantage of the use of a Thomson 
quadrant electrometer, and not only so, but of all the scientific con- 
veniences of Professor Tait’s laboratory. 
The metal to be rubbed was constructed in the form of a circular 
disc (fig. 1), with a projecting tongue for allowing it to be screwed 
on to the brass top (s) of a glass insulator {g) (fig. 2). The diameter 
of the disc was in each case 2*5 inch (6 ‘3 cm.), and the thickness 
two-tenths of an inch (5 mm.). Some of the discs varied slightly 
from that thickness ; but a small difference in thickness does not 
affect the capacity of the disc, for the capacity of a disc depends only 
on the diameter (Clerk-Maxwell, El. and Mag.^ vol. i. p. 222). A 
