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- CHAPTER XXXVIII
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- DETERMINATION OF SPECIFIC GRAVITY
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- 446. Specific Gravity. - The specific gravity or density of a substance
is the ratio of its weight in air to its weight in water at 4°
C. (39.2° F.). In other words, it is the ratio of the weight
of any fragment of a substance to the weight of an equal amount
of water. The specific gravity of a mineral, provided it is pure
and free from inclusions of solids, liquids, or gases, is a constant
quantity. In isomorphous series, or in minerals whose chemical
composition differs in different specimens, there is, however,
a variation, and this serves as a means of separation.
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- The determination of specific gravity properly
belongs to the province of mineralogy and not to petrography,
for which reason the usual methods will be little more than mentioned
here. For more detailed descriptions the student is referred
to the standard works on mineralogy.'
. . .
- 448. Jolly Balance. -
In the Jolly* balance (Fig. 7I7) the specific gravity is
 determined by noting the amount of lengthening of
a spring when the mineral is placed in the upper pan in air (w),
and the amount when it is in the lower pan and immersed in water
(w' ). Here [the specific gravity (G ) is represented
by the equation]
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- G =
w / (w-w' )
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- * P. Jolly: Eine Federwage zu exacten Wägungen.
Sitzb. Akad. Wiss. München, 1864 (I) 162-166.
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- An improved form of jolly balance was designed
by Linebarger,* and another by Kraus.**
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- * C. .E. Linebarger: A new form of the spiral spring balance.
Physical Review, XI (1900), 110-111.
- ** Edward H. Kraus: A new Jolly balance. Amer. jour.
Sci., XXXI (1911), 561-563.
- Idem: Eine neue Jolly'sche Federwage zur Bestimmung des
spezifischen Gewichts. German translation of preceding. Centralbl.
f. Min., etc., 19I1, 366-368.
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