154 7 - PHYSICS, 
interposed body on the table, thus forming the circuit. To melt or heat a 
fine wire, it is made to form part of the circuit in connecting the two rods. 
In the same manner and with this instrument wood and paper may be 
perforated ; resin, alcohol, ether, &c., inflamed, &c., &c. The action of 
the electric spark in the discharge of a Leyden jar is much more powerful 
than that from a prime conductor. By the term striking distance is 
understood the space through which the discharge spark passes in the 
discharging ; it is the measure of the charge in the jar, since the striking 
distance of a jar is proportional to the density of the electricity accumulated 
in it. 
The electricity of a Leyden jar penetrates to a slight distance below the 
surface into the glass. as may readily be shown by experiments on a jar 
with movable coatings (pl. 20, fig. 41). Charge as usual, remove the 
coatings, and replace these by fresh ones. On applying the discharging 
rod, the jar will be found to be charged nearly as high as if communication 
had been made between the original coatings. This experiment shows 
conclusively that the charge resides in the glass, the metallic coatings 
serving only to limit the inductive action. Asa further proof of this, it is 
found that after a large jar has been discharged, a second and feebler 
discharge may be obtained after a short time, the interval being 
necessary to allow the residuary electricity in the glass to pass to its 
surface. 
The action of the condenser, an apparatus for accumulating feeble 
electricity, depends for its principle upon electrical induction. This 
consists essentially of a gold leaf electrometer (pl. 20, fig. 26), with the 
upper plate covered with a thin layer of varnish. Upon this is laid a 
second plate similarly coated and provided with a glass handle. Call the 
upper plate A and the lower B. Ifa conductor charged with a very feeble 
degree of electricity be brought in contact with B, a portion of the fluid 
will be given off, and this will cause the decomposition of that in A. 
Touch A with the finger, and the electricity similar to that of B will 
be given off. The electricity remaining being now of the opposite kind, 
exerts such an attraction on the electricity in B as to permit this to receive 
an additional charge. In this way an accumulation may take place 
in B, which will be shown by the divergence of the leaves on removing 
the upper plate. ° 
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4. Of Electrical Light and the Motions of Electrified Bodies. 
Electrical light is visible only when electricity is in motion, or the 
equilibrium of the fluids is disturbed. The greatest accumulation of 
electricity, under other conditions than these, is unaccompanied by the 
phenomenon. Electricity passes off spontaneously from the angles and 
points of electrified bodies, the appearance of the accompanying light 
‘ varying with the kind of fluid. A current of positive electricity from a 
point exhibits the form of a brush (pl. 20, fig. 44); negative electricity 
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