-  I

GELATINOUS AND ALBUMINOUS INJECTION-MASSEs 259
Combine this vehicle with one of Robin's colouring-masses
(ante, 383,.384, 385, 386, 387).
This mass is not susceptible of change through long keeping.
390. Gelatin Injection-mass (Carmine) (Ranvier's formula').
-Take 5 grammes Paris gelatin, soak it in water for half an
hour, or until quite swollen and soft; wash it; drain it; put
it into a test-tube and melt it, in the water it has absorbed,
over a water-bath. When melted add slowly, and with con-
tinual agitation, a solution of carmine in ammonia, prepared
as follows :-2) grammes of carmine are rubbed-up with a
little water, and just enough ammonia, added drop by drop,
to dissolve the carmine into a transparent solution.
When the carmine has been added to the gelatin you will
have about 15 c.c. of ammoniacal solution of carmine in
gelatin, if the operations have been properly performed.
This solution is to be kept warm on the water-bath, whilst
you proceed to neutralise it by adding cautiously, drop by
drop, with continual agitation, a solution of 1 part of glacial
acetic acid in 2 parts of water. (When the mass is near
neutrality, dilute the acetic acid still further.) The instant
of saturation is determined by the smell of the solution, which
gradually changes from ammoniacal to sour. As soon as the
sour smell is perceived, the addition of acetic acid'nust cease,
and the liquid be examined under the microscope. If it con-
tains a granular precipitate of carmine, too much acid has been
added, and the mass must be thrown away.
Ranvier states that by practice the operator learns to attain
to perfect neutralisation almost infallibly in this way; and
that this is the only way to attain to it. Trust must not be
put in certain formuhe that profess to indicate the propor-
tions of ammonia and acetic acid necessary for neutralisation,
on account of the variation in strength of the solutions of
ammonia kept in laboratories. The method proposed by Frey
of determining beforehand the quantity of a known acetic
1 ' Trait6 technique,' p. 116.

1 " .