England's drying closet, Fig. 75, is simply a light-proof box with wires stretched acroos the interior to support the articles to be dried, e. g. photographic plates. Through the centre runs a 1-in. gas-pipe, open at both ends, with a small gas-jet burning inside it at the lower end. At the top and bottom of the box 2 draught holes are cut, to which a tin tubing of about 3 in. diameter is attached. The gas-tube gets warmed with a very small jet of gas burning in it, a mere pin-hole being sufficient exit for the gas. This warms the air in contact with the tin tube, and also slightly the air inside the cupboard. The consequence is, that a current of slightly warm air is set up, and circulates amongst the plates while supported on the wires, and the drying of the films takes place rapidly. Some 5-6 hours is a sufficient time in which to dry the plates, whilst without the gas-jet it would take 24 hours or more. In the inside of the cupboard, and near the top and bottom, are placed 2 cardboard discs to stop the possibility of any stray light entering, and as the whole affair is placed in the dark room, the chances of any such access even without it would be small.
Inside the cupboard door is a thermometer, and the jet is regulated so that a temperature of about 70° F. is indicated - 80° would do no harm to the plates; beyond that temperature, it might not be safe to go. The small gas-jet used is the same as may be seen in tobacconists' shops; the hole in the end is plugged up, and a very small hole is drilled at the side.
Another photographer adopted a large zinc case with a lid of the same material. He cut a long opening at one end of the bottom, and had another bottom soldered inside with an opening at the opposite end (Fig. 76). He then bad a so-called Russian chimney fastened on one of the sides, and fitted this with a gas-flame placed as shown, so that it might produce the necessary current of air. To make the cover fit air- and light-tight was rather more difficult.
This, however, he managed in the following manner. He had a rim soldered all round in the shape of a gutter, the edge of the lid sinking into the bottom of the gutter, and then filled the latter with small shot, and thus obtained a most perfect closure. This box has been in use ever since, and, with the addition of a wooden tray, and of an iron vessel full of calcium chloride, has done very good service. In the figure, a is the zinc case; 6, gutter filled with shot; c, wooden tray; d, calcium chloride vessel; 6, Russian chimney.
1, Either the temperature in the upper and lower parts is different;
2, or the temperature differs with the duration of heating;
3, or it can only be raised to a moderate degree;
4, or, finally, it can be kept up only by a relatively large consumption of gas.
Meyer proposes to remove these defects in the following manner:
Equality of temperature may be attained by applying the heat at the side - never below - and by taking care that the flame never comes in actual contact with the metal. The space to be heated is to be surrounded with the hot products of combustion of the flame mixed only with the smallest possible excess of air, in such a manner that a triple layer of heated gases, proceeding from without inward, -surrounds the inner mantle. Besides, the outer, or hottest layer, must be protected from too rapid cooling by applying a suitable coating of bad conductibility for heat.
Equality of temperature for any length of time may be best attained by a regulator constructed on the principle of Andrea's, which- contains, in a small, confined space a small quantity of a liquid having a boiling-point a trifle below the degree of temperature to be maintained. The author prefers the modified form suggested by Kemp, and improved by Bunsen, which is wholly constructed of glass except the lower end of the gas-tube, this being made of perforated sheet-platinum.
In order to fill it, the gas-tube a, Fig. 78, is temporarily replaced by a tube b drawn out at both ends and reaching down into the reservoir of the regulator (top of Fig. 78). The lateral branch c is now connected with the vacuum pump, the whole inverted (as in Fig. 78), and the contracted end dipped, first for a moment into the liquid to be used as regulator, and then into mercury, until the chamber is almost, but not quite, full. The apparatus is now turned over, a little more mercury poured in, and the gas-tube c is inserted. When using the apparatus, the gas-tube is first drawn upwards, and, when the proper temperature has been nearly reached, pushed down into the mercury until the supply of gas is reduced to a minimum. By cautious adjustment, it is easy to find the position at which the tension of the vapour developed in the tube raises the column of mercury sufficiently to just close the orifice of the tube c at the proper temperature. As the air-bath cools off very slowly, but heats up rapidly, it is of advantage to adjust the regulator to a slightly lower temperature than actually required.
It is best to have a series of such regulators, charged with substances, the boiling-points of which are about 30° C. apart, and to keep them in a proper receptacle for use. Suitable substances are, for water-baths: ethyl chloride, ether, carbon disulphide, mixtures of ether and alcohol, pure alcohol, benzol; for air-baths: water, toluol, xylol or amylic alcohol, cymol or oil of turpentine, aniline or phenol, naphthalin, diphenyl or diphenylmethane, dipheny-lamine, and perhaps also anthracene. It is not at all necessary to use these in a pure state, particularly those which are solid at ordinary temperature, since they melt more easily when impure. Only very little of solid substances should be introduced, for the excess distils off, and may clog up the gas-tube.