The mixing vessel--a porcelain kettle capable of containing twenty
liters, made at the Royal Porcelain Factory at Berlin, whose products
are unequaled for chemical purposes--is also the boiling vessel, and,
therefore, fits tightly, by means of the tin ring with the wooden
handles, on to a large water bath. The light-tight metal lid, which
can be permanently affixed to the kettle, then supports a stirring
arrangement of fine silver, which dips into the emulsion and has blades
formed like a ship's screw.
The arrangements for injecting the silver vary. The simplest consists of
a large glass vessel containing the silver solution, which is closed
by a glass stopper, and terminates below in a funnel running to a fine
point. This funnel-shaped bottle fits into an opening specially made for
it in the lid of the kettle, and while revolving sends a fine stream
into the gelatine. When it is wished to interrupt it, it is only
necessary to raise the glass stopper in order to see the stream dry up
after a short time.
Another arrangement consists of a contrivance constructed on the
principle of the common India-rubber inhaling apparatus, and sends
the silver solution into the gelatine in the form of the minutest
air-bubbles. After the emulsion is boiled in such a kettle it is allowed
to stand until cool, when the ammonia is added. With such a great
quantity of emulsion and so large a water bath sufficient heat is
retained as to allow the action of the ammonia to take place. As soon
as the time set apart for that reaction has elapsed the water bath is
emptied and filled with pieces of ice and iced water, and the kettle
replaced in it.
If the stirring apparatus be now set in motion, even this large quantity
of emulsion will stiffen in at least an hour and a half. It may be
further remarked that, the outside of the kettle being black, the lid
being light-tight, and all the apertures in it being firmly closed,
nearly the whole process can be conducted by daylight, from the mixing
to the stiffening, so that it is very convenient to be able to keep the
emulsion in the same vessel during all these operations.