The volatile oil, on the presence of which the odour and the taste of plants in a considerable degree depend, is elevated during distillation with water; and a portion of it being retained in solution, the water thus acquires the odour and taste of the vegetable with which it is distilled. The qualities, however, thus acquired by water, are scarcely, in any case, sufficient to give it much power as a remedy; thence the distilled waters are generally employed merely as elegant vehicles for the exhibition of more active substances.

1 Cornu cervi ustum, P, L. 1787.

Waters distilled from aromatic plants are more grateful when the plant is used in the dried state; but when delicate odorous flowers or herbs are employed, and the water acquires little more than odour by the distillation, the vegetable should always, if possible, be used in the recent state. Much care is required, in conducting the process, to prevent any of the vegetable matter from being scorched, and to stop the distillation before the water is tainted by empyreuma. Notwithstanding, however, every attention which can be given, distilled waters, when newly prepared, have a very disagreeable empyreumatic odour, to dissipate which the vessels holding the waters must be left open to the air as long as any of the unpleasant odour remains; but afterwards it is essential for the preservation of the waters that they be preserved in closely-corked vessels.

When long kept, many of the distilled waters undergo a species of decomposition: they become slightly sour, and a ropy viscid matter forms in them, owing to the essential oil which they contain undergoing decomposition. The addition of the spirit is intended to prevent this change from taking place, but it is not adequate to the effect intended; and a much preferable mode is to redistil the waters, after which they will keep good for several years.

Several of these waters are prepared on a great scale, of a superior quality, and cheaper than any the apothecary can prepare.

Aqua Distillata. Lond. Distilled Water

"Take of water, ten gallons. First distil two pints, which are to be rejected, and then distil eight gallons. Preserve the distilled water in a glass bottle."

Edinburgh.

"Let water be distilled in clean vessels, until two thirds of the quantity employed have distilled over."

Dublin.

"Take of spring water, twenty pounds. Put them into a glass retort, and having rejected the first pound which comes over, let one gallon be distilled over with a gentle heat."

Syn. Eaudistillee (F.), Einfaches distillirtes wasser (G,) Gemeenes Destillert Water (Dutch), Watten (Swed.), Acqua Distillata (I.), Agua Distillada (S.).

Water is almost universally diffused over the surface of the earth, but it is not found perfectly pure in any place, which is owing to its great solvent powers enabling it to take up a portion of many substances with which it must come into contact, in its natural state. These impregnations, however, in spring and in river water, are not sufficient to give it any very sensible taste, or to render it unfit for the ordinary purposes of life; but for many pharmaceutical purposes it is necessary that the water be absolutely free from every foreign ingredient. Rain and river water are the purest kind of natural water, but they nevertheless contain a portion of carbonic acid gas, and minute quantities of carbonate of lime and of chloride of calcium : in spring water, besides these ingredients, is found a small portion of chloride of sodium; well water, which is spring water obtained from a greater depth, holds in solution a much larger portion of carbonic acid, and several earthy salts, the principal of which are sulphate and carbonate of lime. By distillation, water is freed from these ingredients, and rendered nearly pure; but it is tasteless and vapid, owing to the absence of the air which it contains in its natural state.

The process should be conducted slowly, with a moderate degree of heat, and not continued longer than the time specified in the formulae, otherwise a minute portion of the saline matter contained in the natural water passes over in the distillation.

Although the necessity of distilled water for many pharmaceutical operations is very obvious, yet, as it is not always easy for the apothecary to prepare distilled water, rain water, filtered through alternate strata of well-washed sand or powdered flints, and charcoal, will answer every purpose for which distilled water is required. Soft water is a more powerful menstruum of vegetable matter than hard water; and resinous substances cannot easily be mixed with water containing calcareous matter, even when mucilage is used, whereas they readily mix with very soft or distilled water. Perhaps it should be a rule to use filtered rain water only in all pharmaceutical operations. In extemporaneous prescriptions distilled water is often ordered when there is no necessity for its use, and often neglected to be ordered when it is absolutely necessary. It may, therefore, be useful to know that it is necessary in formulae containing any of the following substances:-Acidum Sulphuricum, Acidum Citricum, Acidum Hydrocyanicum, Antimonii Potassio-Tartras, Argenti Nitras, Cuprum Ammonio-Sulphas, Ferri Sulphas, Ferri Potassio-Tartras, Ferri Iodidum, Hydrargyri Bichloridum, Liquor Ammonite, Liquor Plumbi Acetas et Diacetas, Liquor Potassae, Solutio Chloridi Barii, Vinum Ferri, Zinci Sidphas.