NEUTRAL CARMINE, ALUM, AND PICRO-CARMINE            55
minate of ammonia. When the free ammonia has evaporated
small bubbles appear, and the solution takes a brighter red
tint.) The solution is left to cool and settle, and by filtering,
the bright red deposit (which may be used over again), is
separated from the neutral dark fluid, which by the addition
of chloral hydrate can be kept for a long time."
This preparation has over simple carmine solutions the
advantage of keeping well, and some others. " If -the solu-
tion is mixed with 4-6 times its volume of strong alcohol a
scarlet-red precipitate is formed. This is separated by filtra-
tion, washed, aud dried, or made into a paste with alcohol in
which some glycerin and chloral is dissolved. Both the
powder and the paste can be kept several months unchanged;
they dissolve easily in water, particularly the paste. The
solution passes readily through the filter, whilst the ordinary
carmine solution can only be filtered with difficulty; it also
keeps a long time unchanged, especially with the addition of
1-2 per cent. of chloral, and it has a much more intense
colouring power.
" By dissolving the carmine powder in a concentrated solu-
tion of neutral picrate of ammonia a combination is obtained
which has all the advantages of ordinary picro-carmine without
any of its disadvantages."
60. Alum-carmine (Grenacher's formula').-An aqueous
solution (of 1 to 5 per cent. strength, or any other strength
that may be prefered) of common or ammonia alum, is boiled
for ten or twenty minutes with - to 1 per cent. of powdered
carmine. (It is perhaps the safer plan to take the alum solu-
tion highly concentrated in the first instance, and after boil-
ing the carmine in it, dilute to the desired strength.) When
cool filter.
This stain must be avoided in the case of calcareous struc-
tures that it is wished to preserve.
61. Alum-carmine and Acetic Acid.-I learn from MI.
1 ' Arch. Mik. Anat., xvi (1879), p. 465.

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