Syrups are liquid medicines, of a viscid consistence. They are solutions of sugar alone, or sugar mixed with honey and dissolved in water, wine, vinegar, or diluted acetic acid. Simple syrup is a solution of sugar in water, and forms the basis of many medicated syrups.
Medicated syrups are those in which one or more medicinal agents enter into the solution, and are commonly prepared by incorporating sugar with vegetable, aqueous or spirituous solutions, expressed juices, etc. When the active principles of the ingredients are dissipated or decomposed by boiling water, or where they are not readily taken up by water, they are frequently dissolved by alcohol, specific gravity 0.935, the alcohol being retained or evaporated subsequently, as may be required; sometimes a tincture of the medicinal agent or agents is added to simple syrup. The stability of a syrup depends on its composition and consistence, the temperature, and the amount of exposure to the air. As many syrups are used in chronic diseases during the absence of febrile or active inflammatory symptoms, the addition of an alcoholic tincture is not then objectionable, unless it be in large proportion; but syrups prepared for febrile or inflammatory difficulties should be entirely free from any spirituous liquor whatever.
If the sugar be in too small amount the syrup will ferment; if in too large amount the sugar will crystallize. The heat employed should be adapted to the character of the active principle; if it be volatile or easily decomposed a gentle heat is required. If it be not injured by heat, concentration may be conducted with a brisk fire, and effected as rapidly as possible. After the syrup has cooled, if a pellicle forms upon its surface, it has been concentrated too much. "Various means have been devised for the preservation of syrups; a little sulphate of potassium, chlorate of potassium, bisulphite of calcium, or sugar of milk, has been recommended for this purpose. One fluid drachm of Hoffman's Anodyne to the pint of syrup will effectually check a tendency to fermentation. The maintenance of a syrup in a regular degree of temperature, say 55° to 60° C. (131° to 140° F.), will tend very much to lessen its liability to ferment. As a general rule, syrups intended to be kept should be bottled while hot, securely corked and sealed, and after cooling should be shaken, that the moisture condensed on the cork may be mixed with the syrup, and not form a diluted layer at the surface" (Mohr and Redwood).
Salicylic acid has of late years been employed to preserve syrups, but if prepared by the cold process to be hereafter mentioned no preservative is required.
Nearly all the compounds that have been proposed for syrups, such as the Eclectic Alterative, Scrofulous, Stillingia, and Pulmonary Syrups, comprise substances whose medicinal principles are imperfectly soluble in water. The process laid down under the formula for Compound Syrup of Aralia (which see) is applicable to this class.
In all these preparations, the principle should be adopted of confining the boiling and evaporation to the weaker portion of the solution, so that those delicate principles which are evaporated or decomposed by heat may be submitted to its action as little as possible.
Owing to the fact that syrups which have been boiled for a long time, and syrups which contain acids, are liable to have a part of their sugar changed to inverted sugar, the cold process of Orynski (1871) has attracted considerable attention, it having been observed that such syrups are less liable to ferment, and that alkaline solutions of copper are more slowly affected by them than when heat has been employed. This process, as employed later by Hunstock, consists in placing the sugar into a cylindrical cone-shaped percolator, in the lower end (neck) of which has been introduced a piece of moistened sponge or cotton, packed neither too tight nor too loose, a cork being inserted into the lower orifice of the vessel. The menstruum, prepared as directed by the official process, is now poured upon the sugar, and the whole is set aside until a portion of the sugar has dissolved and the mass has settled to half its original bulk. The cork is then removed and the syrup allowed to pass drop by drop. Should the liquid fail to dissolve all the sugar, pour upon the final remainder a portion of the filtrate, which will usually complete the solution. This process is especially adapted for syrups made of plants holding volatile principles, or for those whose ingredients are likely to be injured by heat.
Syrups in which vinous fermentation has begun, which may be known by the frothiness due to escaping carbon dioxide and by the vinous odor, may, unless the fermentation has passed beyond the incipient stage, or unless their virtues depend partly or wholly upon volatile principles, be restored by boiling and straining them. Almond and acacia syrups excepted, alteration of non-volatile active constituents seldom takes place at such a stage of decomposition. Glycerin has been proposed as a menstruum for syrups which, when made of sugar, readily decompose. In some cases sugar is used as a preservative on account of its quality of preventing chemical changes. Thus syrup of iodide of iron and Vallett's iron mass both contain sugar for this quality. Syrups, both simple and compound, once constituted an important part of the Eclectic physicians' remedies. But they have been discarded almost wholly, the burden of sugar being of no use in disease and often harmful. In Eclecticism syrups are practically obsolete; yet we give in the pages to follow the formulae of those peculiar to our school.