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minations, especially if the temperature of the water is no higher than that of the ordinary living-room, say 60° F. (15.6° C.)
In
case an accurate balance cannot be obtained, a beam balance described
by Professor Penfield, and shown in the accompanying cut, can be
constructed by almost any one. This gives sufficiently accurate results
for all practical purposes. It consists of a beam of wood supported on
a fine wire, or needle, at o. This must swing freely. The long arm, oc, is divided into inches and tenths, or into any decimal scale, commencing at the fulcrum, o; the
short arm carries a double arrangement of pans, so suspended that one
of them is in the air and the other in water. A piece of lead on the
short arm serves to almost balance the long arm; and, the pans being
empty, the beam is brought
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to a horizontal position, marked on the upright, near c, by means of a rider, d. A
number of counterpoises of different weights are needed. These may be
pieces of bent wire, or bits of glass tube, with a wire hook fused into
one end, g, some of them containing one, two, three, or more
shot, so as to give a variety of weights. The beam being adjusted by
means of the rider, d, the stone or mineral which it is desired
to test is placed in the upper pan, and a counterpoise is chosen,
which, when placed near the end of the long arm, will bring it into a
horizontal position. The weight, Wa, of the mineral in air, is
given by the position of the counterpoise on the scale. The mineral is
next transferred to the lower pan, and the same counterpoise is brought
nearer the fulcrum, o, until the beam becomes again horizontal, when its position gives the weight, Wiv, of the mineral in water. Then Wa divided by Wa—Ww, gives the specific gravity.
A
quick, convenient, and accurate means of separating minerals, and
especially cut stones, according to their specific gravity, is afforded
by use of the so-called heavy liquids. The employment of these depends
upon the fact that a substance will float upon the top of a liquid of
greater density than itself, will remain suspended in a liquid of
exactly the same density as itself, and will sink to the bottom of a
liquid of lower
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