(Figs. 112, 113). In order to avoid the inconvenience of constant watching, to prevent them becoming dry, the water-bath with

"constant level" (Figs. 114, 115, 116) is preferably employed. In this, the water enters through B, flows through the horizontal tube

FIG. 119.

Iron Sand-Baths.

Cinto the bath; this when filled to a certain height, causes an overflow in B which is drawn off by A. By connecting this with

FIG. 120.

Filling Sand-Bath.

a hydrant, the supply of water can be automatically regulated. Fig. 116 illustrates another design in which the water is supplied from a self-regulating flask. Fig. 118 illustrates a large copper

bath heated by a steam jacket; this is adapted for evaporating large quantities of liquids, as in the preparation of extracts.

A Sand-bath (Fig. 119) consists of an iron vessel with either round or flat bottom, which is filled with clean, dry sand. The vessel to be heated is partly embedded in the sand and the entire apparatus placed on a tripod (Fig. 120) and heated. We employ the sand-bath where we desire to heat a body to a high temperature; it also prevents a too rapid rise or fall of temperature which might fracture the vessel. Sea-sand answers best for this purpose, but it should be well washed and dried before use. Not more than a inch layer of sand should be between the bottom of the vessel and the flame.

The Oil-bath is intended for temperatures not rising above 250° C. (482° F.). For this purpose paraffin is best adapted, since most fixed oils evolve very unpleasant odors when heated. Glycerin may be employed for temperatures up to 165° C.,(329° F.).

Baths of Saline Solutions are occasionally employed in operations where we desire a certain regulated temperature, without the precaution of a thermometer. The boiling point of distilled water is 100° C. (212° F.), but if we add any inorganic salt, the boiling point will be raised in proportion to the quantity and nature of the salt added. If we form saturated solutions we find their boiling points constant. The following table gives the temperature obtained by boiling saturated solutions of

FIG. 122.

Air-bath.*" Dr. H. Fleck recommends the simple air-bath, illustrated by Fig. 122, which has been in successful use for several years, doing all the duty of a water-, oil-, or paraffinbath. By a circular cut, or other means, glass rings are prepared from cylinders of various sizes. These rings, from 2 to 10 Cm. in height, are set upon an iron plate, and covered with a similar one, having suitable openings for receiving a thermometer and the vessels intended to be set upon it. If high temperatures are required, low cylinders (of about 2 Cm. in height) are selected; low temperatures require higher cylinders (5 to 10 Cm.). The cylinders are scratched with a diamond, in a vertical direction, so that, if they should crack, the fracture would always be up and down. This simple apparatus permits the maintenance of constant temperatures of 50 to 300° C. (122 to 572° F.) and over. Its transparency is an additional advantage, when it is of importance to watch the progress of reactions, as often happens, in synthetical experiments."

Fleck's Air-Bath.

*As described in the "Proceed. Amer. Phar. Ass'n," 1882, p. 53.

CHAPTER VI.

DISTILLATION.

Distillation is a process whereby a liquid, by means of heat, is converted into a vapor, and this in turn condensed to a liquid by

FIG. 123.

Simplest Form of Distillation.

means of a properly arranged cooling apparatus. This process is resorted to as a means of purification, or for the separation of

the more volatile from less volatile liquids. The apparatus (Fig. 125) consists of a distilling flask (or retort) A, in which the fluid is heated; the cooling apparatus B, in which the vapors are

condensed, is called the condenser, and the receiver C, which serves to collect the condensed liquid. The distilling flask may

FIG. 126.

FIG. 127.

Fractionating Flask with Thermometer.

Flask with T-Tube for Fractionating. be either a retort as illustrated in Fig. 123, or an ordinary flask fitted with a bent tube serving as a beak (Fig. 125), or we may employ the "fractionating flask" (Fig. 126), which admits the use