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.

II.--Digestive Apparatus

It is very desirable that those who do not prepare their emulsion by boiling, but by prolonged digestion, should possess a regulator which will keep the temperature at a given point. Such an apparatus would also be very useful for warming the emulsion for the preparation of plates, as then one would have no further occasion to pay attention to the thermometer and gas stove. In the accompanying diagram a simple contrivance is shown. The gas which feeds the stove passes through a narrow glass tube, a b, into the wider tube, c d e, which is made air-tight at e. This latter tube has an exit tube at f, by which the gas is supplied to the gas stove. At e it is hermetically closed, and at its deepest part it contains mercury, upon which a little sulphuric ether floats in the hermetically-closed limb, e.g. Lastly, there is a minute opening in the narrowest tube at i. The whole apparatus, or, at least, the under part of it, is dipped into the water bath warmed by the gas boiler. It acts thus: As the temperature rises the ethereal vapor in the shorter limb expands and drives the mercury up the longer tube until it closes the opening of the narrow tube, a b, and thereby impedes the power of the stream of gas. Still, the Bunsen burner does not go out, being always fed by the small opening, i, with sufficient gas to support a small flame until the water bath has so far cooled as to leave the opening at b free, when the burner again burns with a strong flame. By removing the cork, c, from the tube the temperature of the water bath is raised, while by pushing it in it is lowered. The apparatus never goes wrong, and is very cheap. It was first made by Herr C. Braun, of Berlin.

II Digestive Apparatus 299 9a

FIG. 1.