684 Proceedings of Royal Society of Edinburgh. [sess. 
science, a thing which I find more and more difficult as I grow older, 
but the more prominent points in it are easily grasped and retained. 
It is found that when two different substances are rubbed 
together they are said to be in a state of electrification. I need 
not describe this state, as all my readers doubtless know it, but 
shall proceed to explain it. What, then, should happen to bodies 
composed of atoms with a constitution such as the bubble atom 
when such bodies, if of different substances, are rubbed together % 
The constituent particles of the atoms of different substances 
having different velocities must, when they are pressed and 
rubbed together, transfer some of their speed from the one 
substance to the other. The substance with the slowest moving 
particles gains speed for its particles, and that with the quickest 
moving particles loses speed for them. What the one gains 
the other loses, the loss of the one being exactly equal to the 
gain of the other. This I take to be positive and negative 
electricity, that body being positively electrified whose particles 
have gained speed, and that whose particles have lost speed being 
negatively electrified.* 
5. The Neutral State. 
The atoms of every element have their own peculiar con- 
stitution ; they may be very massive with a comparatively small 
and dense surface, or they may be of small mass with a com- 
paratively large and open-grained surface ; or they may be massive 
with a large surface, or of little mass with a small one, or in any 
state between these. All that is essential is that their constituent 
particles should have more or less motion according as their 
surfaces are large or small, rare or dense, as explained already. 
How the atoms with the swiftest moving particles will, other 
things being equal, create a greater disturbance in the ether by 
the particles’ greater centrifugal tendencies, sending it, or tending 
* Two substances of the same kind rubbed together may be electrified if in 
different physical states. For instance, rough and smooth glass. The ex- 
planation is, that in rough glass a greater proportion of the surface of each 
atom is free from its fellows, and therefore contains a greater quantity of 
motion than that of those of the smooth glass ; the latter is therefore posi- 
tively electrified, and the former negatively so. 
