Energy, which is the product of these factors, may also be considered as the product of several other pairs of factors, such as
| A Force | × | A distance through which the force is to act. |
| A Mass | × | Gravitation acting through a certain height. |
| A Mass | × | Half the square of its velocity, |
| A Pressure | × | A volume of fluid introduced into a vessal at that pressure |
| A Chemical Affinity | × | A chemical change, measured by the number of electro-chemical equivalents which enter into combination. |
If we obtain distinct mechanical ideas of the nature of electric potential, we may combine these with the idea of energy to determine the physical category in which 'Electricity' is to be placed.
36.] In most theories on the subject, Electricity is treated as a substance, but inasmuch as there are two kinds of electrification which, being combined, annul each other, and since we cannot conceive of two substances annulling each other, a distinction has been drawn between Free Electricity and Combined Electricity.
Theory of Two Fluids
In the theory called that of Two Fluids, all bodies, in their un-electrified state, are supposed to be charged with equal quantities of positive and negative electricity. These quantities are supposed to be so great that no process of electrification has ever yet deprived a body of all the electricity of either kind. The process of electrification, according to this theory, consists in taking a certain quantity P of positive electricity from the body A and communicating it to B, or in taking a quantity N of negative electricity from B and communicating it to A, or in some combination or these processes.
The result will be that A will have P+N units of negative electricity over and above its remaining positive electricity, which is supposed to be in a state of combination with an equal quantity of negative electricity. This quantity P+N is called the Free electricity, the rest is called the Combined, Latent, or Fixed electricity.
In most expositions of this theory the two electricities are called 'Fluids,' because they are capable of being transferred from one body to another, and are, within conducting bodies, extremely
