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Evaporating Dishes.-These are made of porcelain, glass, or platinum, shallow vessels of the appearance shown in Fig. 55. For practical purposes the porcelain dish is generally preferred, being more durable and comparatively inexpensive. Glass evaporating dishes are too fragile for continued use, and are chiefly used in crystallization. Plati

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num evaporating dishes are used in some chemical operations, their expense precluding general use.

In some cases of evaporating directions are given to "reduce" the liquid "to a certain bulk," and for this purpose graduated evaporating dishes have been introduced (Fig. 56).

Instead of same, however, a home-made contrivance answers just as well, and this consists of a rod or cylinder of wood, measuring the volume by marking same on the stick (Fig. 57).

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Fig. 57.-Evaporating dish, graduated with stick.

To avoid the necessity for these graduated evaporating dishes the United States Pharmacopoeia directs such evaporations to be conducted until the liquid is brought to a definite weight, in which case the evaporating dish is transferred from the source of heat to the scales. Of

course, in order to learn the weight of the liquid the weight of the evaporating dish itself, "the tare," must have been previously ascertained. It is a good practice to tare each dish when obtained, and mark the tare in lead-pencil on the outside.

An important point to remember in the use of an evaporating dish is that same should never be heated in direct flame, the source of heat being modified either by use of the wire gauze or by disks of asbestos.

A warm evaporating dish should never be placed directly on a counter. If the counter is of wood, the heat may be sufficient to soften the varnish, and if it is of metal or marble, the rapid chilling of the evaporating dish, through the difference in its temperature and that of the surface of the counter, may cause a fracture of the dish. It should, therefore, be placed on a mat, or, better, on a "grommet," the latter consisting of a rubber ring the construction of which is shown. in appended sketch (Fig. 58).

Stirrers. The stirring of an evaporating liquid is a valuable aid to rapid evaporation; hence the use of stirrers. Their construction depends on the uses for which they are intended. Thus, in large operations, such as evaporating infusions of drugs containing no acid, a large tin spoon is as valuable as a stirrer. In other large operations, such as making syrups, a wooden paddle answers. For the more delicate operations of pharmacy and chemistry glass rods are required, which can be obtained in various diameters from chemical

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Fig. 58.-Grommet.

Fig. 59.-Breaking glass rod.

apparatus manufacturers in three- or four-foot lengths, and then can be readily broken by scratching with a file and applying pressure on either side of the scratch, thus fracturing same, as shown in Fig. 59. In cutting these stirring rods it is best to make them of such length that about two-thirds will rest within the container for which intended.

If more than a third projects over the outside, they are apt to be top heavy. After breaking the rod to the appropriate length, the rough edge should always be smoothed by rotating in a Bunsen flame until slight fusion is effected. If the rough edges are permitted to remain, there is danger of scratching the interior of the evaporating dish.

Fig. 60.-Fume-closet.

Fig. 61.-Hood.

Fume-closets and Hoods.-Evaporation of a large quantity of liquids, and particularly the evaporation of liquids yielding disagreeable or corrosive vapors, in a closed room is objectionable, and to carry The off these vapors fume closets and hoods have been devised. principle of these two is the same, both being an inclosed space connected with a flue or chimney, through which there is sufficient current of air to draw off the offensive vapors. The closet is a large piece of

furniture constructed of wood and glass, and attached directly to the wall (Fig. 60). For a hood can be utilized appropriate lengths of stovepipe, the lower end of which is arranged around a metallic dome, directly under which is placed the evaporating liquid. Hoods work best if connected with a suction pump (Fig. 61).

Vacuum Apparatus. The principle of the operation of a vacuum apparatus having been explained, it is necessary here merely to state that many organic substances, when evaporated even at water-bath temperature, will dissociate. Thus, if sugar is evaporated even at the

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heat of 100° C., the brown substance, molasses, is produced with accompanying vitiation of the finished product. For all such cases vacuum evaporation is desirable, and this is performed in a suitable evaporating utensil, usually heated by the steam coil, and connected with a dome apparatus which, in turn, is connected with an appropriate vacuum pump (Fig. 62). From the illustration it will be seen that the vacuum pan is connected with a worm condenser (p. 97), thus permitting the recovery of the solvent, if it is costly. This is usually the case when vacuum evaporation is employed in pharmaceutical manufacturing, where it is chiefly used in the concentration of alcoholic percolates, such as in making extracts (p. 276). As seen in the illustration, the air is exhausted by means of a pump operated by steam-power. In the technic such evaporating apparatus is largely used, and for pharmaceutic and chemical purposes a small and comparatively inexpensive apparatus of the same character has been devised (Fig. 63).

For removing atmospheric pressure, any form of air-pump can be used, but usually in laboratory work the water suction pump (Fig. 64)

is employed. This can be attached to the ordinary hydrant, provided the water pressure is sufficient. In this simple and useful apparatus the rapid flow of the water through a narrow point draws with it the air from the tube and from the connected aspirator, and if the water pressure is sufficient, a vacuum of 17 mm. can be readily obtained. By a vacuum of 17 mm. is meant that the height of a column of mercury supported by the atmosphere is reduced from 760

mm. to 17 mm.; the latter figure, therefore, showing a pressure of only of the ordinary atmospheric pressure. The indication of this pressure is accomplished

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Fig. 63.-Pharmaceutic vacuum pan.

Fig. 64.-Water suction pump: a, Watertube; b, air-tube.

Fig. 65.-Manom

by the instrument called the manometer. Some forms of manometer are delicate apparatus made on the the principle of the steam gage, but for chemical work a simple manometer consisting of a bent tube (Fig. 65) filled with mercury, and set up against a scale, answers all purposes.

The simple form of such vacuum evaporating apparatus is that figured in vacuum distillation (p. 104).

DISTILLATION

Distillation is the process of converting a liquid into a gas, and condensing the gas back into a liquid.

Objects. In distillation we have an admirable method of purifying volatile substances. If such a liquid contains impurities less volatile than itself, the impure product is placed in a still and heated, whereupon the substance desired will distil, leaving the less volatile substance behind. In many cases the foreign matter is water, in which case distillation is of great value in strengthening the product under consideration. This is particularly the case in a separation of alcohol from whisky or other alcoholic liquors, which is always accomplished by distillation. The liquid thus purified by distillation is called a "distillate."

In other words, distillation is evaporation of a liquid and subsequent condensation of its vapor. Evaporation has been described

on a preceding page, and here it is necessary to turn attention only to the second part of the process, namely, condensation.

As already learned, to convert a liquid into a vapor means the expenditure of considerable energy, which energy is obtained from a source of heat, this heat being converted into the other form of energy,that necessary to keep the molecules of the gas separate from one another, is no longer indicated by means of the thermometer, and such heat is called "latent." If the converting of the liquid into a vapor means the expenditure of considerable heat, so in the inverse operation of changing the vapor back into a liquid an equal amount of heat is given off. In other words, heat is rendered latent in converting a solid into a liquid or a liquid into a gas, and, in turn, latent heat is rendered sensible by changing gas back to liquid, or liquid back to a solid form. Therefore to condense a vapor back to liquid form the chilling of the vapor is necessary, that is, the latent heat must be

Fig. 66.-Charging plain retort.

absorbed by being brought into contact with a cooling agent. The converting of the vapor back to the liquid form in condensing can be accomplished in many cases by bringing the vapor in contact with chilled water.

The amount of water required can be figured out mathematically, and depends on the temperature of the vapor and the temperature of the water before and after its use as a condensing agent. Thus, 1 kilo of steam at 100° C. will take 23.2 kilos of water at 20° C. for condensation, and the temperature of the water after condensation will be 45° C., or 1 kilo of steam at 100° C. will take 24 kilos of water at 10° C., heating it to 35° C. during condensation.

The apparatus required for distillation consist primarily of a boiler, in which the liquid is vaporized, and the condenser, in which the vapors are chilled until they are condensed to the liquid form. This combination of boiler and condenser is commonly called the still, and the many forms of this apparatus which are in use can be roughly divided into two classes the alembic form and the retort form.

In the alembic form the condensing apparatus is directly over the boiler, while in the retort form the condensing apparatus is placed on one side of the boiler, thus not directly over the source of heat. The retort form of a still being more commonly used, it will be discussed first.

Retort Still. So called because the original form of boiler for such operation was the glass instrument called a retort, which consisted of a glass flask with a long and tapering neck, bent at an acute angle. This simple form of retort is inconvenient by reason of the difficulty in filling such a retort, it being, of course, essential to secure a pure distilled product that the neck of the retort through which the distilling vapor passed should be free from impurities.

Therefore, the charging of such a retort must never be done by pouring the liquid down the neck, but the liquid must be introduced into the bowl of the retort by means of a funnel with a long tube connected therewith (Fig. 66).

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