The telegraph manual: a complete history and description of the semaphoric, electric and magnetic telegraphs of Europe, Asia, Africa, and America, ancient and modern.

FRANKLIN S ELECTRICAL THEORIES. 55
till 1754, his experiments and observations were embodied in a
series of letters, which were afterward collected and published.
" Nothing," says Priestley, " was ever written upon the subject
of electricity, which was more generally read and admired in
all parts of Europe, than these letters. It is not easy to say,
whether we are most pleased with the simplicity and perspicuity with which they are written, the modesty with which the
author proposes every hypothesis of his own, or the noble frankness with which he relates his mistakes when they were corrected by subsequent experiments." The opinion adopted by
Franklin with respect to the nature of electricity differed from
that previously submitted by Dufaye. His hypothesis was as
follows: " All bodies in their natural state are charged with
a certain quantity of electricity, in each body this quantity
being of definite amount. This quantity of electricity is maintained in equilibrium upon the body by an attraction which the
particles of the body have for it, and does not therefore exert
any attraction for other bodies. But a body may be invested
with more or less electricity than satisfies its attraction. If it
possesses more, it is ready to give up the surplus to any body
which has less, or to share it with any body in its natural state;
if it have less, it is ready to take from any body in its natural
state a part of its electricity, so that each will have less than
its natural amount. A body having more than its natural
quantity is electrified positively or plus, and one which has
less is electrified negatively or minus. One electric fluid is
thus supposed to exist, and all electrical phenomena are referable either to its accumulation in bodies in quantities more
than their natural share, or to its being withdrawn from
them, so as to leave them minas their proper portion. Electrical excess then represents the vitreous, and electrical deficiency the resinous electricities of Dufaye: and hence the terms
positive and negative, for vitreous and resinous." The application of this theory to the explanation of the Leyden vial
will appear in its proper place.
Besides this theory, we are indebted to Franklin for the discovery of the identity of lightning and electricity, for the invention of paratonnerres, and for the discovery of induction,
which latter principle was immediately taken up, and pursued through its consequences by Wilke and CEpinus, and soon
led to the invention of an instrument, which in the hands of
Volta, became the condenser, now so useful in electroscopical
investigations.
Franklin's hypothesis was investigated mathematically by
CEpinus and Mr. Cavendish, between the years 1759 and 1771.