gether, and the result is zero.
Franklin proposed to call these electricities positive and negative.
These names are well chosen, but we do not know any reason why one
should be called positive rather than the other. The electricity
generated on glass when rubbed with silk is called positive.

Let us now examine the distinction between positive and negative
electricities somewhat more closely, aiding ourselves by two cases
which are somewhat analogous.

Two air-tight cylinders, A and B, contain air at ordinary pressure.
The cylinders are connected by a tube containing an air-pump in such a
way that, when the pump is worked, air is taken from A and forced into
B. To use the language of the electricians, we at once generate two
kinds of pressure. The vessels have acquired new properties. If we
open a cock in the side of either vessel, we hear a hissing sound, if
a light body is placed before the opening in A it would be attracted,
and before the opening in B it would be repelled. Now this is only
roughly analogous to the case of the electrified bodies, but the
analogy will nevertheless aid us in our study. If the two vessels are
first connected with the air, and then closed up and the pump is set
to work, we increase the pressure in B and diminish the pressure in
A. To do this requires the expenditure of a quantity of work. If the
cylinders are connected by an open tube--a conductor--the difference
in pressure disappears by reason of a flow of gas from one vessel to
the other.

If we had a pump by means of which we could pump heat from one body
into another, starting with two bodies at the same temperature, the
temperature of one body would increase and that of the other would
diminish. If we knew less than we do of heat, we might well discuss
whether the plus sign should be applied to the heat or to the cold,
because these names were coined by people who knew very little about
the subject except that these bodies produce different sensations when
they come in contact with the human body.

Furthermore, we find that whether the hand is applied to a very hot
body or to a very cold body, the physiological effect is the same. In
each case the tissue is destroyed and a burn is produced. Shall we now
say that this burn is produced by an unusual flow of heat from the hot
body to the hand, or from the hand to the cold body, or shall we say
that it is due to an unusual flow of cold from the cold body to the
hand, or from the