HARDENING AGENTS             159

tion ').-Equal parts of 1-400 solution of platinum chloride,
and 1-400 solution of chromic acid. " Prof. Merkel states
that he allowed from three to four days for the action of the
fluid " for the retina; for annelids Eisig employs an immer-
sion of three to five hours, and transfers to 70 per cent.
alcohol; for small leeches Whitman finds " one hour sufficient
and transfers to 50 per cent. alcohol."
209. Bichromate of Ammonia. 2 to 5 per cent. Several
weeks will be necessary in most cases. This and the other
chromic salts take five or ten times as       as chromic acid
to do their work, but the hardening is more perfect, the pre-
parations never becoming brittle. The ammonium salt has
come of late to be generally preferred to the potassium salt.
210. Bichromate of Potash.-Same strength as bichromate
of ammonia.
211. Chromate of Ammonia. 5 per cent. For twenty-four
hours (intestine, Klein).
212. Miller's Solution.
Bichromate of potash      .   2-21 parts.
Sulphate of soda     .    .    . 1     ,,
Water      .    .    .    .     100
The duration of the reaction is about the same as with the
simple solutions of chromic salts.   fZ-P-o  fc  0
213. Potassium Bichromate and Cupric Sulphate.-A
variation of Miller's solution ; instead of 1 per cent. sulphate
of soda you take- 1 per cent. sulphate of copper.         The
hardening properties are superior to those of Miiller's
solution.   This formula is, I believe, now very generally
employed in Germany.      I do not know by whom       it was
suggested.
214. Chromic Acid and Alcohol.-Chromic acid of the
strength given above diluted with half its volume of 95 per
cent. alcohol. (Solutions of chromic acid should always be
1 Merkel, 'Ueber die macula lutea, &c.,' 1870, p. 19. ' Journ. Roy. Mic.
Soc.' (N.S.), ii (1882), p. 871.