A
similar quotient will be obtained whether large or small pieces are
taken for determination, the specific gravity being totally
independent of the actual gravity or weight.
The
determination of the specific gravity of gems or minerals becomes then
a question simply of manipulations by which the relative weights of the
substance in water and air can be obtained in the easiest and most
accurate way.
The
most common and generally the most convenient way of doing this is by
obtaining the weights of the stone in water and air directly by means
of a delicate balance. The stone is first weighed in air and the weight
recorded. It is then put into a holder of fine platinum wire, bent
into a spiral form, and suspended from the arm of the balance. The
length of the wire is such as to allow the stone to become completely
immersed in a vessel of water supported on a stand above the scale pan,
but in such a manner as to allow the pan to swing free. In this way the
weight of the stone and wire in water can be accurately taken. The
stone is then removed, and the wire weighed suspended in the water as
before. The weight of this is subtracted from the previous weight, so
as to remove the weight of the wire from the calculation, and the
remainder is the loss of weight of the stone in water. Dividing the
weight of the stone in air by this remainder gives, as stated above,
the specific gravity. Several precautions need to be taken to insure
accurate results. In the first place, only distilled water should be
used, as ordinary waters have higher density. Again, bubbles of air
often adhere to the surface of the stone, especially if it be rough, or
if it is pervaded by cracks, which would obviously, if allowed to
remain, lessen the weight. These can sometimes fe removed by dipping
the stone in water several times and blowing the water off, or they can
surely be destroyed by boiling for a few minutes the water in which the
stone is immersed, and then allowing it to cool before the specific
gravity is taken.
Strictly
speaking, the specific gravity of a body is its weight compared with
that of water at the temperature of 4° Centigrade (39.2° Fahrenheit),
which is the point at which the density of water is the greatest.
Determinations at other temperatures should, therefore, if absolute
accuracy is desired, be corrected to 4° C. In practice, however, the
error is so trifling that it may be disregarded in all ordinary deter-
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