NEUTRAL CARMINE, ALUM, AND PICRO-CARMINE  53

IIiz is
CHAPTER V.
NEUTRAL CARMINE, ALUM-CARMINE, AND PICRO-CARMINE.
56. Neutral Carmine (Heidenhain's formulal).-A carmine
to   solution is prepared according to Beale's formula (No. 50),
i   but with the omission of the alcohol. This is rendered almost
neS  neutral, either by cautious addition of dilute acetic acid or by
dIQ  driving off the free ammonia by warming on a water-bath.
'U  (The test for approximate neutrality is, that a watch-glassful of
n    the solution, allowed to stand uncovered, should precipitate all
its carmine, through evaporation of the ammonia, in the course
tio  of twenty-four hours.)
ft4     The method of using is as follows:-The sections are placed
othos  in a watch-glassful of the solution, and the watch-glass is
placed in an air-tight vessel together with a second watch-glass
which contains water having a trace of ammonia (just enough
to be perceptible by the smell). This small quantity of
ammonia, gradually evaporating and being absorbed by the
staining solution, suffices to keep the carmine dissolved for
twenty-four to twenty-eight hours, by which time the stain-
-al  ing is generally complete. The sections are washed in com.
L(  mon glycerin, then brought into a watch-glassful of concen-
trated glycerin and exposed for twenty-four hours in a closed
vessel to the vapour of a small quantity of acetic acid.
Mount in glycerin. The stain is elective for certain cells (of
the peptic glands, see the article quoted) ; it is nuclear and
permanent in glycerin. Should an overstain happen, it may
1 'Arch. Mik. Aiiat.,' vi (1870), p. 402.