Figs. 79,80, and 81 show the air-bath as usually employed by the author. It consists of 4 concentric walls of sheet copper, 2 of which are attached to the upper plate, and the others to the bottom plate.
Fig. 79 is arranged for a drying chamber: Fig. 80 for the distillation of substances which easily decompose when coming in contact with (over) heated glass; Fig. 81 for the dry distillation of substances which should not be heated beyond a certain point (f. i., citric acid in the preparation of aconitic acid, &c).
The innermost cylinder* surrounds the space a to be heated, which is closed from below by a double bottom 6, fastened by a bayonet-clamp. The upper cover, also double (the 2 walls being kept parallel by inner supports, of which one is shown at A), has 2 tubulures, one (l) for the insertion of a thermometer, another (•) for the regulator, and another for the escape of the heated vapours. To this cover the 2 cylinders d and f we attached, while e and c are soldered to the bottom piece, which is also provided with 3 legs. The heating is done by a brass ring attached to the legs, with a supply of gas controlled by the regulator i. The ring has holes of 2-3 meters. The little flames thus produced burn quietly, and may be easily regulated. With the same amount of gas which is furnished by .1 gas-cock supplying an ordinary Bunsen's burner, the space in a ( = about 5 litres) may readily be heated to 300° C. and over, even when it is not closed below. But in order to obtain this result, the intervals between the several cylinders, in which the products of combustion circulate, must not exceed 10 millimeters. Besides, the outer cylinder / must be protected with a non-radiating cover.
The best, for this purpose, is a layer of asbestos (in sheet), to be applied so as to leave a little space between it and cylinderf, which space is to be filled out with siliceous earth ("kieselguhr") or mineral wool.
* The air-chambers illustrated above are not square, bat round. The illustrations represent a vertical section through the centre.
If tubes are to be heated, the modification shown in Fig. 82 may be used. It is also here of importance that the channels through which the warm air circulates are very narrow, scarcely 1 cm. apart. The 8 iron tubes pass through the narrow walls, which latter are not double, but covered with little flaps hinging upwards (one corresponding to each tube), as closely as possible fitting to the surface of the outer cylinder, but remaining slightly distant from the ends of the tubes. In case a glass tube (inserted in one of the iron tubes, for being heated) should explode, its fragments are caught by the loosely hanging Haps. Between the iron tubes, a Babu's regulator may be.
For special usee the above forms of air-baths may be still further modified. It is, how-to remember that the heated gases should surround the space to be heated In a triple layer; that the hottest layer should be near the outside, and that the intervals between the walls should admit as little excess of air as possible. The gases escaping above must have the property of extinguishing a glowing splinter of wood.
Edmund Bithler, of Tubingen, furnishes air-baths of the above-described construction.
A much enlarged form of the hot-air closet is widely adopted in curing agricultural produce, such as fruits, tea, etc. Everywhere in Tennessee, North Carolina, and Georgia, one may see at the farmsteads rows of boards tilted up to the sun and covered with sliced fruit of muslin to keep away the insects, and to give the fruit a finer colour. These small lots of fruit are collected by the country storekeepers, and thus find their way to the great cities and a market. The first improvement made in drying fruit was. tried in the North, and consisted in covering the fruit with glass. The hot-bed sash idle in the barn found a new duty. Wooden boxes or frames made to fit the sash were prepared and set upon legs to raise them above ground. Holes were cut at the front near the bottom, and at the back near the top, to secure a current of air through the frame; within these glass-roofed frames the fruit was spread on trays in the full sun-light. The glass kept out rain, birds, and insects, and the fruit dried more quickly and with less labour than in the old way, and with a decided improvement in its appearance. Experiments were also made with stoves. The cooking stove dried the fruit more quickly than the sun, but it was wanted for other purposes. The next step was to erect drying closets.
A small enclosed place or closet of any convenient shape or size was put up in the farmhouse or shed, and in this was set u small stove. The sides of the closet were protected from the fire by brickwork, and above the stove were ranged shelves for the fruit; inlets for the fresh air were made at the bottom, and at the top ventilators were provided for the escape of the heated air and vapour. Such appliances answered a very good purpose, and are often used to save the surplus fruit of a small farm for domestic use or for sale. Besides these domestic appliances there is now in use a very good iron stove or drying machine, dry all kinds of fruit in a much better manner than the wooden closets, which are liable to take fire. This stove is portable, and may be used out of doors.
A fire is kept up in the fire-box at the base, and above it are movable shelves for apples, peaches, berries, corn, grapes, or other fruits or vegetables. A conthe apparatus, sweeping across the trays of fruit and quickly extracting all their moisture. The smoke-flue from the lire passes through the escape for the hot air, aud materially assists the movement of the air. Driers of this form are largely used in the peach districts of the East and the grape-growing country of the Pacific coasts. They are easily managed, and will dry as much fruit in a day as a family can peel and slice in that time.