108 Prof. Thomson, Electrification produced by heating Salts. 
Oxides. 
Ca + 
A1 
(PbO + 
Pb ^Pb.,o 4 + 
(pba + 
BaO + 
BaCb 
FeA 
Fe.,0, 
Bi 2 0 3 + 
SnO + 
SnO., 
+ 
+ 
+ 
+ 
+ 
+ 
+ 
+ 
Nitrates. These give but little electrification when rubbed, 
but the electrification is always negative, the same as that 
produced by heating the salt. 
Chlorides and iodides give slight negative electrification 
when rubbed, the yellow as well as the red form of Hgl 2 was 
tried, both forms become negatively electrified. The carbonates 
and sulphates give very little electrification when rubbed and the 
sign of it is very capricious. 
It will be seen that with the exceptions of the highest oxides 
of a metal electrification by heating gives the same sign as that 
by friction. In the case of the highest oxide the discrepancy is 
due to the fact that this oxide would at the high temperatures 
used be converted into a lower oxide. 
From the agreement between the electrical effects produced 
by rubbing and heating we may conclude that the process by 
which the salt gets electrified is the same in the two cases. This 
suggests that the salts are covered with a double layer of electri- 
fication ; in the case of the phosphates the positive layer is outside, 
the negative inside, while in the oxides the negative is outside, 
the positive inside, and that electrification is produced by the 
partial removal of the outer layer, by rubbing when the electrifi- 
cation is produced by friction and by the heat when the electrifi- 
cation is produced by heating the salt. 
There seems no reason why the presence of the double layer 
at the surface of a body should be limited to the case of salts. 
If it is present in all bodies, then, as I suggested some years ago, 
electrification by friction may always be due to the removal of 
one or both of the outer coatings of the double layers covering 
the bodies which are rubbed together. 
I wish to express my thanks to Mr E. Everett for assistance in 
the experiments with the hot salts, and to Mr W. H. Hayles for 
making the experiments with the cold ones. 
