Bath. In chemistry, &c., a vessel or apparatus containing some medium in which the vessel holding the substance to be heated is immersed, instead of being exposed to the direct action of the fire; by which means a limited and uniform temperature may be ensured.
The highest temperature that can be given to any substance contained in a vessel placed in another of boiling water, is about 205° or 206° Fahr.; but by adding 1⁄5th part of common
salt to the bath, a heat of fully 212° may be obtained. Baths of fusible metal, saturated solutions of salt, sand, and (on the large scale) steam, are also used for the same purpose. A bath of oil may be safely heated to about 500° Fahr. without suffering decomposition, and will be found an exceedingly appropriate and convenient source of heat in many processes. The simplest and most convenient form of water bath is that afforded by raising water to the boiling point in a copper basin placed over a gas lamp, and supporting the vessel to be heated over the basin, by means of a circular hoop of copper resting on the top of the basin. By this means the lower surface of the dish or vessel to be heated is brought in contact with the steam. Copper basins, fitted with a series of concentric copper rings, so as to render the basin capable of supporting dishes of different sizes, are made for this purpose.
For drying many substances an air bath is required. The accompanying cut represents a convenient form of air bath. It consists of a cylindrical copper vessel (A), the cover of which is moveable, and has two apertures, the middle one (E) serving for the escape of vapour, and the lateral one (C) for the insertion of a thermometer. The vessel holding the substance to be heated rests on a ring within the box, supported on a tripod.
A larger air bath, by means of which several small vessels can be heated at once, is seen below.
Air-baths are sometimes surrounded with a jacket, and may be converted into water or oil baths, according as the jacket is filled with either of the fluids. For a Table of Boiling-points, see Ebullition.
An air-bath of constant temperature between 100° and 200° C. has been contrived by Sprengel. It consists of an ordinary hot-water oven made of sheet-lead autogenously soldered, and filled with dilute sulphuric acid boiling at the required temperature.
In order that the temperature may remain constant, the water which distils from the dilute sulphuric acid is condensed and allowed to flow back again into the bath by means of a worm of lead cooled by the atmosphere, or a long vertical metal or glass tube.
