"1st. Those principles only should be decocted whose virtues 'reside in principles which are soluble in water.
"2nd. If the active principle be volatile, decoction must be an injurious process; and if it consists of extractive matter, long boiling, by favouring its oxidizement, will render it insipid, insoluble, and inert.
"3rd. The substances to be decocted should be previously bruised or sliced, so as to expose an extended surface to the action of the water.
"4th. The substances should be completely covered with water, and the vessel slightly closed, in order to prevent as much as possible the access of air; the boiling should be continued without interruption, and gently.
"5th. In compound decoctions, it is sometimes convenient not to put in all the ingredients from the beginning, but in succession, according to their hardness, and the difficulty with which their virtues are extracted; and if any aromatic or other substances containing volatile principles, or oxidizable matter, enter into the composition, the boiling decoction should be simply poured upon them, and covered up until cold.
"6th. The relative proportions of different vegetable substances to the water must be regulated by their nature. The following general rule may be admitted. Of roots, barks, or dried woods, from two drachms to six to every pint of water: of herbs, or flowers, half that quantity will suffice.
"7th. The decoction ought to be filtered through linen while hot, as important portions of the dissolved matter are frequently deposited on cooling; care must also be taken that the filter is not too fine, for it frequently happens that the virtues of a decoction depend upon the presence of particles in a minutely divided state." - Paris's Pharmacologia.
All acid syrups ought to have their full quantity of sugar, so as to bring them to a consistence without boiling-, because the very action of much heat destroys their acidity, and makes them liable to candy; and this more particularly holds good where the infusion or juice, etc, has any fragrancy in flavour, because the volatile oil is dissipated by boiling. The same observation is also applicable to those infusions of flowers which give out their colour, and which is necessary to be retained, such as violets, pinks, etc, as boiling injures them.
Those syrups which are made from decoctions, and do not take a sufficient quantity of sugar to bring them to a due consistence without boiling, require to be clarified so as to render them transparent; but this is often an injury, as the whites of eggs take off some of their chief properties with the scum; therefore, the decoction should first be rendered clear by settling or filtering, and the sugar should be clarified and boiled to the height of the feather or ball before the decoction is added, when it must be reduced to the proper degree.
The best and most general method of making syrups is to add a sufficient quantity of the finest loaf-sugar, in powder, with the juice or infusion, etc, stirring it well until a small portion settles at the bottom, then place the pan in a larger one containing water; this is termed the bain-marie; put it on the fire, and the heat of the water as it boils will dissolve the sugar; when this has been thoroughly effected, take it off and let it cool; if more sugar is added than the quantity above named, it will separate in crystals, and not leave sufficient remaining in the syrup for its preservation. (See observations on Sugar-boiling). When cold, put it into small bottles, fill them, cork closely, and keep in a dry cool place. Be particularly careful that no tinned articles are used in the making of syrups from the juice of red fruits, as it will act on the tin and change the colour to a dead blue.