"Spontaneous evaporation at ordinary temperatures of the atmosphere is easily accomplished with ethereal and similar volatile liquids, but alcoholic tinctures, in order to avoid a too-prolonged contact with air, require a somewhat elevated temperature, that directed by the Pharmacopoeia being usually 50° C. (122° F.). Where a higher temperature exerts no injurious influence upon the important constituents, a water-bath is most convenient for the purpose; and since the evaporating liquid will always remain several degrees below the boiling point of water even though the water in the bath may be actively boiling, empyreuma is effectually prevented. Steam-baths will likewise be useful, provided the arrangements are such that the pressure of steam cannot, or only slightly, exceed that of the atmosphere. Sand-baths and other contrivances by which the temperature is likely to rise above 100° C. (212° F.) should be used only with due precautions. Evaporation is most successful if conducted from a shallow dish, with a current of dry air passing over the surface of the liquid, which may at the same time be agitated; but the use of metallic spatulas for stirring during evaporation, is generally inadmissible; the proper material is porcelain or wood. Various mechancial contrivances have been constructed with the view of saving the manual labor and constant attendance in stirring, and then serve a good purpose". In evaporating a tincture the alcohol is lost, but in many cases it is desirable to recover it, when distillation is resorted to.

Fig. 392.   Water Bath

Fig. 392. - Water Bath.

Fig. 393.   Steam Boiler with Still

Fig. 393. - Steam Boiler with Still.

For use on a small scale Professor Parrish has constructed an apparatus which is shown in Fig. 393. A is a boiler made of thick copper, which may be heated by a furnace or by several Bunsen burners. Water is supplied through the valve H; G is a safety valve; E is a valve for regulating the supply of steam which is carried off through the exhaust valve F. The evaporating pan, made of well-tinned copper, is coupled together with the steam jacket B, upon the flanges of which is fitted the head C, and secured by means of clamps, the joint being made steam tight by placing a coil of lampwick or loose cord between the flanges. If used for distillation, C is connected with a condenser; if for evaporation only, C is removed. By proper regulation at E and F, either a very moderate heat may be applied to the pan or the temperature may be raised above the boiling-point of water.

A very serviceable pharmaceutical still has been constructed by Professor Remington (1879). It is made of copper; the head, which is fastened in the same manner as the preceding, connects with a condenser formed of a combination of several Liebig's tubes, (compare Fig. 394), whereby the condensing surface is very materially increased. The still may be used in a water-bath or over direct fire, and if desired may be convertecr into a steam-bath, by fastening a suitable dish as an evaporating pan upon the body of the still containing some water.

In many processes carried on in the arts, vacuum-stills are used for the evaporation of liquids, the boiling point of which is lowered in proportion as the pressure within the apparatus is reduced to below that of the surrounding atmosphere".

The vacuum process (in vacuo) is a method of evaporating in a closed metallic vessel, from which the air and vapors are removed by means of a steam air-pump. When water is boiled in the open air, its temperature is 100° C. (212° F.), but if it be placed in a strong metallic still, heated by a steam coil, and if the air and steam are "pumped out," the water will boil at a lower temperature. The more perfect the vacuum produced by the pump, the lower the point at which the water will boil. This vacuum process is much used in the large way in making cane sugar and condensed milk, and in some large pharmaceutical laboratories certain things, like plant extracts, malt extracts, etc., are evaporated rapidly at a low temperature, so as to avoid the bad effects of more heat. Also in a large way, there is a vacuum process for preparing fluid extracts. A metallic percolator is connected with a metallic receiver from which the air is exhausted by an air-pump. This allows the pressure of the air above to force the menstruum into and through the drug in the percolator, as is claimed, more thoroughly and more quickly than by simple percolation.

Fig. 394.   Remington's Pharmaceutical Still

Fig. 394. - Remington's Pharmaceutical Still.