MAGNETISM AND ELECTRICITY. 149 
Free electricity can pass from one body to another, provided the latter 
be a conductor, in two ways: by immediate contact, and by transmission 
at a greater distance. In the latter case a spark will be observed to pass 
between the two bodies at the moment of intercommunication. This spark, 
which, under favorable circumstances, may be two feet in length, is 
capable of inflaming alcohol, ether, resin, gunpowder, gun-cotton, &c., as 
well as the inflammable gases. The latter experiment is readily performed 
by means of the electric pistol (pl. 20, fig. 25), which consists of a small 
metallic vessel, closed by a cork stopper, and filled with an explosive 
mixture, as of oxygen and hydrogen. Into the lower part of the tube a glass 
tube, tf, is cemented, and into this is again cemented by sealing-wax, a 
metallic wire ending in two small balls, 6, b’. When the electrical spark is 
communicated to the outer ball, it passes from the other ball to the opposite 
wall of the vessel, inflaming the gas in its passage; by the resulting 
explosion the cork is driven out with a loud report. The electrical mortar 
(pl. 20, fig. 39) acts somewhat differently. Here the electricity produces 
so sudden an expansion in gas or liquid, oil for instance, as to eject a ball 
with great violence. The amount of this expansion may be measured by 
the so-called thermometer of Kinnersley (pl. 20, fig. 40). In its lower part 
there is a liquid which at first stands at an equal height in two intercom- 
municating tubes. The expansion of the gas above the liquid in the larger 
tube, where a spark passes through between the balls, 5, b’, causes its ascent 
in the smaller tube, Zt’. 
2. On Electrical Induction; the Electric Machine; the 
Electrophorus. 
When an unelectrified body is brought near .one that is electrified, a 
separation of the combined electricities of the former takes place, the 
positive occupying one extremity, and the negative the other. The 
..electricity of the second body attracts the opposite kind to its end, repelling 
that of like character with itself to the other end. When the bodies are again 
separated, the decomposed electricities unite, and no sensible trace whatever 
of free electricity remains. This decomposition of electricity in one body 
by another, without actual contact, is said to be produced by electrical 
induction. 
To illustrate the preceding proposition, we may make use of the following 
experiment ( pl. 22, fig. 63). Take a rod of metal with its extremities bent 
into hooks, and fix it horizontally on an insulating vertical stand of glass. 
To each hook suspend two pith balls with strings of some conducting 
material, as linen. Approximate an electrified body, r, to the metal rod, 
and both pairs of pendulums will diverge, showing that they have become 
electric. They collapse, however, on the removal of r. The electricity 
found to exist in the balls is the result of the induction of the body r. 
To determine the kind of electricity in any body, whether positive or 
negative, it is vy 2cessary to make use of an electroscope charged with a 
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