A thin transverse section placed under the microscope
exhibits a series of curvilinear lines diverging from the centre and
interlacing each other with great regularity and beauty, closely
resembling in appearance the engine-turning of a watch. It possesses a
specific gravity varying from 1.888 in the tooth of the walrus, to
2.843 in that of the elephant. Its mean gravity is therefore about two
and a half times greater than water. The best, finest, and most
valuable ivory is that obtained from the African elephant. When
recently cut, it exhibits something of a yellowish transparent tint,
which is due to the oil it contains, but this gradually changes to a
beautiful and permanent white. It is not easily stained or destroyed
by exposure to the atmosphere, and on that account is used in the arts
for all the higher purposes, and especially for carved ornaments--such
as chess-pieces, crucifixes, and articles of _virtu_. Indian ivory, on
the contrary, when first cut, is perfectly white, but it becomes
yellow and discoloured with age and exposure. A good illustration of
this circumstance is presented by the dingy-coloured keys of an old
pianoforte.
This popular definition of good and inferior ivory is however, in
point of fact, somewhat incorrect, since ivory obtained from the coast
of Africa is often much inferior to that obtained from the Indian
Archipelago.
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