INDIFFERENT LIQUIDS          239

Glycerin does not preserve humatoxylin' or cochineal stains.
For the preservation of carmine stains it should be acidu-
lated with 1 per cent. of formic or acetic acid. Glycerin is
useful for preserving (until wanted for section-cutting)
tissues that have been treated with osmic acid, as it prevents
them from becoming brittle as they otherwise do.
The already high index of refraction of glycerin (Price's
glycerin, n = 146) may be raised to about that of crown-
glass by dissolving suitable substances in the glycerin. Thus
the refractive index of, a solution of chloride of cadmium
(CdC12) 2 in glycerin may be 1504; that of a saturated
solution of sulpho-carbolate of zinc in glycerin may be
1-501; that of a saturated solution of Schering's3 chloral
I hydrate (in crusts) in glycerin is 1-510; that of iodate of zinc
i in glycerin may be brought up to 1.56.4 The clearing action
of glycerin may thus be greatly increased, and the full
aperture of homogeneous objectives brought to bear on objects
i mounted in one of the above-named solutions.
The sulpho-carbolate of zinc solution5 may be prepared by
I taking equal parts by weight of Price's glycerin and sulpho-
carbolate of zinc crystals, mingling the two, and applying
E sufficient heat to boil the glycerin. The solution can be
i made in about an hour, but no fear need be had about
I boiling too long, as the longer this is done the less liability
will there be for the solution to deposit crystals on the
I bottom  of the bottle when cooled, which it will do if the
temperature is only kept up long enough to dissolve the
I crystals. Filter while hot. The index may be brought up to
It is generally believed that glycerin is an efficient preservative for
I hanmatoxylin preparations, but I consider that it cannot be relied on to
keep the stain perfect.
2 Journ. Roy. Mic. Soc.,' ii (1879), p. 346.
3 Ibid (N.S.), i (1881), p. 943.
lbid., p. 366.
5 Ibid., iii (1880), p. 1051.

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