cased
by the mounting, is the only surface-portion of paste. In other cases
the whole of the crown is genuine, whilst often both the upper and
lower portions are solid and genuine, the saving being effected by
using a paste centre at the girdle, covered by the mounting. Such a
stone as this last mentioned is often difficult to detect without using
severe tests and desperate means, e.g.:— (a) by its crystalline structure (see Chapter III.) ; (b) by the cleavage planes (see Chapter IV.) ; (c) by the polariscope (see Chapter V.) ; (d)
by the dichroscope (see Chapter VI.) ; («) by specific gravity (see
Chapter VIII.) ; (/) cutting off the mounting, and examining the girdle
; (g) soaking the stone for a minute or so in a mixture said to
have been originally discovered by M. D. Rothschild, and composed of
hydrofluoric acid and ammonia ; this will not answer for all stones,
but is safe to use for the diamond and a few others. Should the jewel
be glass, it will be etched, if not completely destroyed, but if
genuine, no change will be apparent ; (h) soaking the diamond
for a few minutes in warm or cold water, in alcohol, in chloroform, or
in all these in turn, when, if a doublet, or triplet, it will tumble to
pieces where joined together by the cement, which will have been
dissolved. It is, however, seldom necessary to test so fur, for an
examination under the microscope, even with low power, is usually
sufficient to detect in the glass the air-bubbles which are.almost
inseparable from glass-mixtures, though they do not detract from the
physical properties of the glass. The higher powers of the same
instrument will almost always define the junction and the layer or
layers of cement, no matter
