It is especially adapted for the preparation of concentrated infusions and essences, as they may thus be obtained of any required strength, without loss, or requiring concentration by heat, which is so destructive to their virtues.
When ordinary tinctures are made in large quantities, displacement is never likely to supersede maceration on account of any practical advantages it may possess. If the prescribed directions be duly attended to, the process of maceration is unexceptionable. The process is more simple than the other; the mode of operating more uniform; it is, in fact, always the same; it requires less of skill and dexterity in conducting it; it requires less constant attention during its progress, which, in operating on large quantities, is a consideration; and finally, the apparatus required is less complicated. When, however, only small quantities are to be made at a time, and kept in stock, the adoption of the process of displacement will often be found convenient and advantageous. It offers the means of making a tincture in 2 or 3 hours, which, by the other process, would require as many weeks.
The preceding remarks are mainly gathered from Cooley's Cyclopaedia. More recently the subject has received great attention from J. U. Lloyd, of Cincinnati, Ohio, and the results of his observations are thus recorded in the Proceedings of the American Pharmaceutical Association: -
One of the most frequent operations to be performed by the pharmacist is to separate from the crude materials, offered principally by the vegetable kingdom, active principles from others inert or not desirable. This object is reached by bringing the same into the liquid state by solution, with the aid of a proper solvent (menstruum). Thus we have the process of maceration and percolation, the latter being a modification of the former, calling in the aid of gravitation. To arrive at a proper understanding of the laws which govern the solution of substances, that is, the transfer of a solid into the liquid state through the aid of solvents, we should consider first the greatest agent in percolation, - the attraction of gravitation. This unknown force impels all terrestrial bodies toward a common centre, the centre of the earth.
If we arrest the fall of a solid and pour upon it a liquid, that liquid will flow over the solid, excepting a small amount held by adhesion, and will fall from the lower surface towards the earth. If that solid be impenetrable, and insoluble in the liquid, it will remain intact; if soluble, it will gradually assume the liquid state and disappear. If the solid be porous the liquid will enter. This is due to absorption - a molecular force, which is working independent of the attraction of gravitation, and overcoming it to a limited degree, thereby exercising a great influence over the process of solution, beneficial inasmuch as it ensures a closer and more continued contact between the solvent and the solid. Thus, if a certain amount of liquid be slowly poured upon the porous body, we shall find that attraction of gravitation will fail to detach the liquid from the lower side; it does not flow over the outside, but enters, is absorbed, and held within its substance. The attraction of gravitation still exerts itself, for the actual weight of the mass is the sum of the separate weights of the 2 bodies. Without further examination we might suppose the materials at rest; such, however, is not the case.
There are disturbing elements which produce constant motion; thus, an alteration of temperature will excite a change in the relative position of the molecules of the liquid, and temperature constantly changes. But besides the motions of the molecules, caused by the constantly varying changes of temperature, there is osmosis, an attraction that induces currents of liquid through cellular tissue. Gravity, however, overcomes at first all of these various contrary influences - among which we may class diffusion - and is ever tending to draw the liquid most heavily charged with soluble matters downward through the lighter, and thus there seems to be no rest, but, on the contrary, continual change.
The influences mentioned exert themselves whether the solid be large or small, whether a single particle of dust in a quantity of liquid or an innumerable number placed in a mass and covered with liquid. Let us turn our attention to solution. Throwing aside all theories as to the why and wherefore of the change of state from solid to fluid, we must accept the fact, that below the melting temperature certain solids will, to a fixed extent, assume the form of liquids if in contact with particular fluids. The conditions necessary to effect and promote this change are: surface exposed to the dissolving medium, circulation of the liquid, temperature and time of contact between the surfaces of solid and the liquid. In regard to the first of these conditions, it is invariably found that the rapidity of solution increases with the area of the surface exposed; thus, for an example, if a cubic crystal of potassium bromide, or any other substance, 1 in. in dimension, be surrounded with water, the surface in contact with the water will be 6 sq. in. If the crystal be bisected by a plane parallel to any 2 of its sides, the amount of the material remains the same, but its surface has been increased 2 sq. in.
Let each half now be divided into 4 equal parts, and there will be a total of 12 sq. in. of surface, exactly twice the amount of the original cube. Division can be theoretically, and in the above instance according to mathematical laws, continued to the extent of our imagination, and each cube divided into 8 will double the amount of the surface. But in practice we meet with obstacles of various nature which soon interpose insurmountable limits to accurate divisions, making our further efforts in that direction impracticable, and the desired increase of surface is most readily effected by pulverising the solid, thus obtaining irregular surfaces.