Later, in 1792, it was found
by Gray that certain substances possessed the power of carrying;
"conducting" as we now term it; the mysterious fluid from one substance
to another; from place to place. This discovery constituted an actual
epoch in the history of the science, and justly, since this small
beginning with a wet string and a cylinder of glass or a globe of
sulphur was the first unwitting illustration of the net-work of wires
now hanging all over the world. The next step was to find that all
substances were not alike in a power to conduct a current; _i.e._,
that there were "conductors" and "non-conductors," and all varying
grades and powers between. The next discovery was that there were, as
was then imagined, several kinds of electricity. This conclusion was
incorrect, and its use was to lead at last to the discovery, by
Franklin, that the many kinds were but two, and even these not kinds,
but qualities, present always in the unchanging essence that is
everywhere, and which are known to us now by the names that Franklin
gave them; the _positive_ and _negative_ currents; one always
present with the other, and in every phenomenon known to electrical
science.
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