IMITATIONS AND TESTS                   73
	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