I conceive then, that this globe of earth and water, with its plants, animals, and buildings, have diffused throughout their substance, a quantity of the electric fluid, just as much as they can contain, which I call the natural quantity.
That this natural quantity is not the same in all kinds of common matter under the same dimensions, nor in the same kind of common matter in all circumstances; but a solid foot, for instance, of one kind of common matter, may contain more of the electric fluid than a solid foot of some other kind of common matter; and a pound weight of the same kind of common matter may, when in a rarer state, contain more of the electric fluid than when in a denser state.
For the electric fluid, being attracted by any portion of common matter, the parts of that fluid, (which have among themselves a mutual repulsion) are brought so near to each other by the attraction of the common matter that absorbs them, as that their repulsion is equal to the condensing power of attraction in common matter; and then such portion of common matter will absorb no more.
Bodies of different kinds having thus attracted and absorbed what I call their natural quantity, i. e. just as much of the electric fluid as is suited to their circumstances of density, rarity, and power of attracting, do not then show any signs of electricity among each other.
And if more electric fluid be added to one of these bodies, it does not enter, but spreads on the surface, forming an atmosphere; and then such body shews signs of electricity.
I have in a former paper compared common matter to a spunge, and the electric fluid to water: I beg leave once more to make use of the same comparison, to illustrate farther my meaning in this particular.
When a spunge is somewhat condensed by being squeezed between the fingers, it will not receive and retain so much water as when in its more loose and open state.
If more squeezed and condensed, some of the water will come out of its inner parts, and flow on the surface.
If the pressure of the fingers be entirely removed, the spunge will not only resume what was lately forced out, but attract an additional quantity.
As the spunge in its rarer state will naturally attract and absorb more water, and in its denser state will naturally attract and absorb less water; we may call the quantity it attacks and absorbs in either state, its natural quantity, the state being considered.