NEUTRAL CARMINE, ALUM, AND PICRO-CARMINE             61
has formed is removed by filtration, and the picro-carmine
is fit for use.
Carnoy finds this picro-carmine gives better results than any
other; it has kept for more than two years without change in
his laboratory. He prefers it to Hoyer's.
71. Picro-carmine and Eosin (Lang's formula').-Fifty
parts 1 per cent. picro-carmine and 50 parts 2 per cent.
(aqueous) solution of eosin. Objects are left in the mixture
from half a day to four days, according to their permeability.
The picrin is then extracted by frequently changed washes
of 70 per cent. alcohol; this is followed by 90 per cent. alcohol,
which is changed until no more eosin dissolves out.
A double stain, inserted here because the eosin appears to
play a peculiar role, its superior penetrating power enabling it
to serve as a vehicle for carrying the picro-carmine through
structures which would otherwise be impervious to the latter.
This method was found to be the best of all for Dendrocela
and other Platyhelmia.
72. Picro-carmine (Weigert's formula).-Two grammes of
carmine are soaked for twenty-four hours (in a spot protected
from evaporation, in 4 grammes of ammonia; 200 grammes of
concentrated solution of picric acid are then added, and the
whole put away for twenty-four hours more. Small quantities
of acetic acid are then added " until the first slight precipitate
appears even after stirring." The whole is again put away
for twenty-four hours more, when it will be found that there
has formed a precipitate that can only partially be removed
by filtration; ammonia is then added drop by drop at inter-
vals of twenty-four hours, until the solution becomes clear.
If the solution stains too yellow, acetic acid is added; if it
overstains red, a little ammonia is again added. All badly
staining samples of picro-carmine may be improved in the
same way by addition of acetic acid.
1 'Zool. Anzeig.,' 1879, p. 45.
2 'Virchow's Archiv,' Bd. 84, pp. 275, 315. ' Zool. Jahr.,' 1881, p. 40.