of Edinburgh, Session 1833 - 84 . 
427 
give a positive deflection about the magnitude of the above, and 
left untouched till next day. When tested next day it gave a read- 
ing about +52, and after considerable whisking the positive 
deflection vanished and changed to negative. The result of this 
experiment favours the idea that the positive electricity is due to 
the existence of minute particles on the surface of the disc. 
On 20th December further experiments were made to elucidate 
the effect of rubbing with emery paper and with sand paper. The 
rubbing paper was attached to the end of a glass rod. 
Emery Paper. 
1 
Sand Paper. 
1 
Copper, 
+ very slight. 
+ more than with 
emery. 
Tin, . 
0 
- 
Iron, . 
+ more than copper, 
less than zinc. 
0 
Zinc, . 
+ 
0 
Lead, 
+ less than zinc. 
+ more than with 
emery. 
I scratched the surface of the zinc disc with emery paper ; when 
the scratch was rubbed with the brush positive electricity was ob- 
tained, but after the scratches were rubbed a few times the electricity 
became negative. This experiment indicates with what care ob- 
servations must be made in this subject in order to obtain definite 
results. 
The fourth series of observations were taken up with gold, silver, 
nickel, bismuth, antimony, aluminium, magnesium, German silver. 
The first three are in the form of discs of brass electro plated. The 
aluminium and magnesium are cut out of thin sheet metal, and are 
screwed on to a disc of brass. The electro^plates were on the 20th, 
21st, and 27th compared with the disc of copper, but on the 28th 
with a similar electro-plate of copper. It was found that the copper 
electro-plate gave only two-thirds of the deflection given by the 
copper disc ; the entries for the previous days have been corrected 
by multiplying by |, the other discs are compared with the disc 
of copper. 
On 20th December the electro-plates were rubbed with clean 
chamois leather before beginning, and they were whisked a few 
