removal of atmospheric pressure (page 74). We find a practical application of this fact in the "Vacuum Apparatus." It is of the greatest value in pharmacy, for the rapid concentration of certain solutions, chiefly of organic substances, such as the preparation of solid extracts. By means of this, we not only increase the rapidity of evaporation, but avoid injury to the sensitive plant principles by the lower degree of heat employed

and absence of the oxidizing influence of the atmospheric air. Fig. 106 illustrates a smaller vacuum apparatus for the use of apothecaries. Its construction and use is exceedingly simple. The copper vessel A, serves as a water or steam bath; in this is suspended a porcelain or copper evaporating dish, which is covered with a glass dome B made air-tight by means of a rubber joint; the thermometer is inserted at C, the air is exhausted from the apparatus by means of the water-pump

E, the glass vessel D serves as receiver, and this is closed by a metallic lid to which is attached the indicator F, enabling the operator to control the vacuum. Fig. 107 illustrates a sectional view of a vacuum pan of modern construction, such as is employed in the industries in the evaporation of sugar solutions, or the preparation of extract of beef or of condensed milk.

RAPID EVAPORATION.-Besides the usual methods of facilitating evaporation as already mentioned, another deserves mention, and that is the method of evaporation by the application of heat over the surface of the liquid. This method is applicable to all non

inflammable liquids such as are not injured by heat. Fig. 108 illustrates Hempel's method, in which a flame burns directly above the surface of the liquid, the entire apparatus being enclosed in a cylindrical glass vessel, which is connected with a pump, for the rapid removal of the vapors and gases. Another method, is that of the Government laboratory of the Netherlands. The apparatus consists of an iron plate standing on four legs; near each corner is an opening, through which the tubes of four Bunsen burners are adjusted to such a height that their tops will be a little higher than the edge of the capsule, which is placed on the iron plate. A second plate is then adjusted over the burners. When the four flames are lit, the upper iron plate causes the flame to spread out along the plate, the direct flame and radiation of heat causing a rapid evaporation to take place. In evaporating inflammable liquids on the water bath, it is best to surround the flame by a

wire gauze (Fig. 109), to prevent any danger of the liquid or vapor igniting.

For evaporating or concentrating small amounts of very volatile solvents, such as ether, carbon disulphide, or petroleum-ether, it is best to place the vessel in a drying closet or desiccator, or under a bell jar which contains an absorbent, such as sulphuric acid, burnt lime, etc. For when we allow these fluids to evaporate in the open air, owing to the lowering of temperature, and the rapidity of evaporation, water is condensed on the sides of the vessel and contaminates the residue. Evaporating vessels of perpendicular sides should be employed for volatile fluids containing solids in solution, as such liq

uids tend to crawl up over the edge of the dish and deposit solid matter on the outside.

FIG. 108.

FIG. 109.

In evaporating liquids which give off irritating or corrosive vapors or gases, the operation should be carried on in a "Draught Chamber" (Fig. 110). This may be built into a chimney-flue or immediately in front of it, the flue leading off from the top of the chamber as shown in Fig. 111. A gas flame above, serves to create a strong, steady draught. The chamber may be arranged as shown in Fig. 110, where the draught chimney extends up one side of the chamber, there being an opening below, where the gas may be lit. Above, near the top, is a similar opening which serves to remove all gases. In absence of such arrangements, we can remove noxious vapors by inverting a large glass funnel over the evaporating liquid, it being held in position by means of a clamp over the neck, a rubber tube being attached to the end of the stem, and the other end of the tube being led to a water pump. The suction produced will carry off all traces of noxious vapors.

Hempel's Evaporator.

In evaporating such liquids as contain salts in solution, it is necessary to stir constantly, for, as they become concentrated, a saline scum forms upon the surface, which prevents further evap

oration. Those liquids which form a scum or skin on the surface (for instance, milk), or frothy solutions, should be mixed with washed sand or pulverized glass and then constantly stirred while evaporating.

SPONTANEOUS EVAPORATION.-Water, and many other liquids, will evaporate without the application of heat, simple exposure to the air, with as large an area of surface as possible, being all that is necessary. We apply this method of concentration to those liquids which contain bodies that are injured by any degree of

heat, also for the slow vaporization of solutions of crystallizable bodies, so as to obtain well formed crystals. Instead of placing these vessels in the open air, we sometimes place them in a drying closet (Fig. 172), always taking the precaution to cover the vessel loosely with muslin or filter paper, to prevent particles of dust from dropping into the fluid. We sometimes carry on slow evaporation, by placing the dish on a tripod and setting this upon the flat top of a stove or over a hot-air radiator.

BATHS. For general purposes of evaporation we employ various baths, in order that we may control the temperature of evaporation. Sometimes it is necessary to subject a liquid to a prolonged

high degree of heat; again in others it is necessary that the temperature should not rise above a certain limit. For the purpose of regulating the degree of heat, we use various liquids of different

boiling points. The most common of these is water (used in the water-bath), by means of which we reach a temperature of about 97° C. A water-bath is generally constructed of copper or iron