stones. Sonstedt's solution is a saturated watery solution of the double iodide of mercury and potassium of sp. gr. 2-77
; it mixes with water without any marked change in the volume, and
hence may have its specific gravity lowered in proportion to the
quantity of water added. The boro-tuugstate of cadmium has also been
used. More recently, however, methylene iodide has been used ; this is
a carbon compound of the formula CH2I2, and
has a density of nearly 3"33 at 15° C. ; owing to its high coefficient
of expansion it is important to use it at a definite known temperature
; it has the advantages of mixing freely with benzine (sp. gr. -88)
; of being light-coloured, so that the mineral under test can be easily
seen ; and mobile, so that the test specimen can move freely. By means
of benzine or methylene iodide or a mixture of the two, any density
between '88 and 3'33 can be obtained. Further, by saturating methylene
iodide with iodine and iodoform its density can be raised as high as 3-6.
The principal gem stones with a higher specific gravity than 3'6 are
Corundum, Spinel, Chrysoberyl, the Garnets, Zircon and Cyanite. The
most convenient way of using these solutions is to have them in four
wide-mouthed stoppered bottles of glass. No. 1 contains the saturated
solution of iodine and iodoform in methylene iodide, and has a sp. gr.
of 3'6 ; No. 2 contains pure methylene iodide, sp. gr. 3-3 ;
No. 3 a dilution of methylene iodide with benzine to sp. gr. 3.0 ; and
No. 4 a similar, but further, dilution to sp. gr. 2"65. The specific
gravity of these solutions must be tested from time to time by some
such convenient means as the use of specific gravity beads, which are
hollow glass beads so weighted that they neither float nor sink in a
liquid of a certain
