Students will find it advantageous, in their reading about tinctures and fluid extracts, to distinguish between those which mix clear with water and those which do not. First, they should note the constituents of the drugs and the menstrua employed to exhaust them. Here they will learn if the active constituents are water-soluble, in which case a weak alcohol or water alone is the menstruum used to exhaust them, the function of the spirit being to preserve the preparation resulting. Tincture of opium is a good example of these. They form clear mixtures with water. Secondly, students will observe and collate a group of preparations which contain resins, essential oils, and other water-insoluble active substances. These can be divided into two broad classes : (1) Preparations containing resins, with little other dissolved material, so that on adding water to them a milky mixture is formed, in which the resinous particles more or less rapidly coalesce, and the active substance is thus completely precipitated. Tincture of Indian hemp is a good example of this class.

It is impossible to dispense them without a suspending agent or emulsifier. (2) A second class consists of most of the fluid extracts containing resinous bodies, tinctures of gum-resins, and aromatic or resin-containing drugs which are made with the weaker alcohols. These contain extractive matter, which, under certain conditions, suffices to prevent the precipitated particles from coalescing, and so maintains the emulsified condition. Preparations of this latter class do not usually require mucilage, especially as they are frequently prescribed with other ingredients containing extractive or saccharine matter; and if in any case an alkali is present, this may suffice to saponify enough of the resinous substance to make a perfect emulsion. The rule to follow generally in regard to the compounding of resinous fluids in aqueous mixtures is: Mix with the water everything except the resinous fluid, which add last and gently shake. If, for example, it is a mixture like this:

Put into the bottle the glycerin, spirit, and 6 ounces of water; shake; now pour in the tincture in a thin stream without touching the neck of the bottle, shake gently, and make up. Custom sanctions the use of a long-necked funnel to introduce the resinous tincture into the centre of the mixture so that it can be shaken up with the other constituents before any of it in undiluted condition has come into contact with the side of the bottle. The paper cone mentioned in connection with the next mixture serves the same purpose and saves the trouble of cleaning the funnel neck.

In some cases it is advisable, should a strong alcoholic preparation be prescribed along with the resinous one, to mix both before adding to the water, the object being to get the precipitated resinous particles as small as possible. For the same reason the resinous preparation is poured into the largest possible volume of aqueous fluid. Tinctures of guaiacum and Indian hemp cannot be dispensed in aqueous mixtures unless mucilage or a similar substance is used. The best modus operandi is to take a measure of acacia mucilage equal to that of the tincture, dilute it with as much water as possible, pour in the tincture, and gently agitate. When much water has to be added, the finished mixture should not contain less than mxx. or 3ss. of mucilage to the ounce. Syrups, even if present in considerable quantity, do not effectually prevent deposition of resin. The following is another example:

A good mixture may be made as follows: Mix in a dry measure the tincture of cannabis with the sal volatile and other tinctures. Dissolve the potash salt in about 4 ounces of water, and add the wine. Now make a small cone of white paper with a small opening at the apex. Immerse the apex of the cone just beneath the surface of the bicarbonate solution (which may be held in a 4-ounce measure), and shoot in the tinctures through the cone, which must be quickly and simultaneously more deeply immersed ; remove it, stir the mixture, bottle, and finish. Only slight precipitation takes place after several hours, but the dispenser should take into consideration the vicissitudes which the mixture will undergo during the following six or eight days, and remember that the last dose should be as nearly as possible like the first. It is, therefore, better in the last case to stir 3ij. of mucilage with 3vj. of water, and add the cannabis tincture gradually while stirring; dilute to 3 ounces, add the remaining ingredients, the potash being dissolved in a little water and added last.

The following is a recent prescription containing a resinous extract:

The simplest procedure to make a nice mixture of this is to place the extract on the bottom of a mortar, warm the mortar over a water-bath until the extract becomes soft, add to it 80 grains of powdered acacia, and rub well and firmly till the extract is all taken up by the acacia and a dampish uniformly green powder results; then add the chalk mixture a little at a time with steady trituration till the acacia and the extract are equally distributed. The result is a nice green mixture free from dark specks, which are usually the trouble in such mixtures.

With large quantities of resinous tinctures or concentrated mixtures of them the addition of mucilage is essential, as in the following: