140 PHYSICS. 
To render this now visible picture permanent, the plate must be immersed 
in a weak solution of hyposulphite of soda, to dissolve such iodide of silver 
as has been unacted on by light. After gently washing the plate in water, 
and allowing it to dry, the operation will be completed. Instead of the 
silvered plate, paper may be employed as the medium, and various other 
substances used besides the salts of silver. The former process is called 
the Daguerreotype ; the latter the Talbotype or Photograph. This latter 
term is sometimes employed to denote all pictures produced by the 
chemical action of light. 
Fig. 74, pl. 21, represents the form of the daguerreotype camera, as at 
present used; fig. 75 is an enlarged view of the tube containing the 
achromatic objective. The mercury box for bringing out the image is 
seen in fig. 76. The best cameras are furnished by Voigtlander of 
Vienna, instruments of his construction being more used than any other, 
especially in the United States. 
MAGNETISM AND ELECTRICITY. 
A. Maenetism. 
1. On the Mutual Influence of Magnets on each other, and on 
Magnetic Bodies. 
A magnet is a body possessing the property of attracting and adhering 
to iron, and of being attracted by it. This attraction is termed magnetism 
or magnetic force, and its cause is generally considered to be a peculiar 
imponderable agency, conventionally termed the magnetic fluid. Magnets 
are divided into two kinds: natural, as in the magnetic oxide of iron (or 
loadstone), and artificial. Every magnet has on its surface a line or space 
where there is no attraction; this is called the neutral line of the magnet. 
The two portions into which the magnet is divided by this line, are called 
its poles, although the poles are generally understood to indicate those two 
opposite extremities of the magnet where the attraction is strongest. If a 
magnet be dipped into iron filings, it will attract them to some points and 
not to others; these points about which the filings accumulate are the 
poles. Pl. 20, fig. 1, represents this phenomenon for a natural, and fig. 2 
for an artificial magnet ; in both mm’ is the neutral line where there is no. 
attraction. The intensity of attraction, as indicated by the quantity of the 
adherent filings, decreases from the poles to this central line. The 
experiment is best made by laying a piece of stiff paper on the extremities 
of a horse-shoe magnet; on sifting fine filings upon the paper, over the 
poles, they will arrange themselves in regular curves, as seen in fig. 8, 
the influence of the magnet thus extending through the paper. 
By suspending a bar magnet horizontally from a thread, and approximating 
a second magnet, it will be seen that each pole of the latter attracts one 
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