There are many industries and operations where the removal of excess moisture from solid bodies is an important feature. Convenient means of effecting this object will now be described. The apparatus employed may be divided into two classes, air-ovens and water-ovens, according as heated and dried air or hot water is the active medium.

Air-Ovens

The ordinary steam or hot-air chambers for laboratory use, although meeting the most of the requirements for which they are designed, have the disadvantage of being more adapted for experimental than manufacturing purposes. The want of a cheap and convenient apparatus induced Maben to bring under notice a design (Fig. 73) due to Hislop, one of his apprentices, who intended it for drying photographic gelatine plates; but, by slight modifications of the interior, it is perfectly adapted for the purposes of the laboratory.

The chamber consists of a strong wooden box a, 18 in. high by 18 in. wide, and 14 in. deep. To the front a door is attached, hinged in this instance, but a vertical sliding movement would.

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The bottom of the box has a perforation of 3 in. diameter into which a Zinc cylinder b is securely fitted, and to this is soldered the upper end of a copper cone o with a fist bottom, while into this latter a bent tube d 2 1/2 in. diameter, and 9 in. total length, is securely inserted in the manner shown. A corresponding perforation is made in the top for receiving a tube to answer the purposes of a chimney.

Tiling a Bunsen burner or a spirit lamp as the source of heat, the flame is directed to the bottom of the cone o with the remit that the heated air ascends into the chamber, being diffused by means of a dispersion board h about 4 in. square, which is placed over the orifice. At the end of the tube d is fitted a "hit-and-miss" regulator g, which consists of a series of triangle-shaped holes, with a revolving disc behind, so that the size of the apertures can be increased or diminished, thus enabling the amount of air entering to be under partial control. The highest temperature to which the air in the chamber has been raised is 180° F. (82° C.) which is sufficiently high for moat operations. If a uniform temperature of say 100° F. (38° C.) be required, the admission of air must be regulated accordingly by means of the regulatory, accuracy being ensured by the insertion of a thermometer m into a perforated cork fitted into a 1/2-in. aperture on the top of the chamber. By this means, there is no difficulty in keeping within 2 1/2 less or more of the desired temperature.

If a rapid current of warm air is desired, this can be had by placing an angular tube i on the top of the chimney e; by heating the angle of the tube, a draught is quickly created.

It is desirable in some cases to filter the admitted air; this can be done by stretching a piece of lint or other suitable material between the regulator g and the tube d, by which means dust particles are effectually excluded.

The metallic parts of the apparatus being made to screw of and on, they can be detached at will, so that we can thus have a aeries of wooden chambers suited to different purposes. In this instance, the chamber being Intended for drying gelatine plates, it was of course constructed so that the light would be effectually shut out, but it is obvious that a small glass window would add greatly to its value for moat other purposes. The advantages of this chamber are its simplicity, its perfect security against overheating, and its small cost, - it can be made for a few shillings. It is light and easily handled and is always ready for work, a current of pure hot air being obtained in a very few minutes after the application of the Bunsen flame. It is specially adaptable in the preparation of granular and scale compounds, for drying precipitates, hardening pills previous to coating, and in other operations requiring a current of hot air.

Another writer describes his drying-closet as being made of teak 1 in. thick, with light-tight door in front; the ends project beyond the bottom to form legs; the top and bottom are both double (4 in. apart), and the air enters through a slit 3 in. wide, and reaching right across the box. This slit is at one end, and the air has then to pass along the double bottom to the other end, where it gets into the box through a similar slit, thus keeping out the light; and it gets out at top in a similar way. Over the exit at top is fitted a tin or copper chimney 3 ft. high, in which burns a Silber lamp, giving a good draught, and drawing a large quantity of air through. Inside the box are brackets (each having a levelling screw through it, with the point upwards), projecting from the ends, on which are laid plate-glass shelves cut the width of the box, but 3 in. shorter, so that when the shelves are in place, if one is pushed close to the right end of the box and the next to the left, and so on, the air has to pass backwards and forwards over the plates. His box has 3 shelves, 13 in. wide and 32 in. long, and will dry 6 photo-graphic plates 15 in, by 12 in., or, of course, anything less that will lie in the same space.

Some have an arrangement for drying and warming the air before it enters the box; but this sometimes induces blisters and frilling. Shelves should be far enough apart to get the hand in easily, say 6 in.

Fig. 74.

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Fig. 74 shows a sectional view of another form of photographic drying-box, a are shelves on which to put plates. In the drawer b are placed some lumps of calcium chloride. This absorbs moisture very rapidly, and the air in passing through it is thoroughly dried. In the flue d is a small gas-burner, and below is a light trap c, made of tin. The gas-jet is for the purpose of causing an extra current of air to pass over the plates. It is better to confine the plates as much as possible to the 2 middle shelves, as there they are sure to be safe. At e is a sketch showing how the door of the box should be rebated into the side.