702 Proceedings of Royal Society of Edinburgh. [sess. 
oxygen, or negative ion, of the acid, presumably, containing a 
good deal more,* acts first of all by induction, or, in other 
words, by excess of pressure on the outer atoms of the zinc, 
driving the motion, or electricity, inwardly, and so filling up, 
or tending to fill up, the collapse of the inner portions of the 
zinc atoms, and detaching, or tending to detach, them from one 
another. The partially detached zinc atoms now having a 
greater quantity of electricity on the side furthest away from 
the oxygen, the excess of pressure on that side and on the 
opposite side of the oxygen over that in the space between will 
drive the oxygen towards the zinc, with which it then unites. 
In the process of union a greater collapse takes place between 
the zinc and the oxygen than existed between the oxygen, 
or negative ion, and the hydrogen of the acid ; some of the 
electricity of the collapsed portion, or, in other words, the energy 
of position of the collapsed particles, is used up in separating 
the oxygen and zinc from their former partners, and should there 
he no path provided for the remainder to escape, it comes 
away in heat. Should, however, a path he provided, as in a 
galvanic battery, the surplus energy comes away as a “ current ” 
of electricity. 
29 . I have come to the conclusion that hydrogen, the metals, 
and the “ positive elements ” generally have the greatest surface 
density, or close-grainedness, because they always come away 
from combination with a positive charge. On the other hand, 
oxygen, chlorine, etc., the negative elements, have the least 
surface density. This seems to he confirmed by the great specific 
gravity of the positive elements compared with that of the 
negative. It will follow, then, that the particles of the positive 
elements, when in the neutral state, move with the least speed, 
and that when they (the positive elements) enter into combination 
* The sulphion of sulphuric acid is negative to the hydrogen with which 
it was united after the latter comes away from it, but before their union the 
sulphion was positive to the hydrogen, and in the act of union parted with 
some of its electricity to it, which made the hydrogen positive and the 
sulphion negative. Similarly the sulphion, before union, is positive to 
the zinc with which it unites, and during the act of union makes the zinc 
positive, leaving itself far more negative than when it parted with its 
hydrogen. This can be explained by the fact that its union with the 
zinc is far closer than it was with the hydrogen. 
