From all that has preceded the compounder will have little difficulty in formulating a theory of emulsification. It may be put thus: If we shake a fixed oil with water, we break it up into a multitude of minute particles, so that the entire mixture has a milky appearance, but if allowed to stand the oil speedily separates. This is owing to two causes- the lower specific gravity of the oil and the want of adhesion between its particles and those of the water. The art of making emulsions consists in finding and introducing some body which will keep the particles of oil in the minutest possible size, and suspend them in the vehicle, so that when at rest the mixture shall not separate. The emulsifying media vary, as has been seen, but the principle of action is practically the same in every case. The object is to break up the substance to be emulsified into minute particles, and to enclose each of them in a coating of the emulsifying agent. The following examples are instructive:

Make a thick mucilage with the gum and a little water, then add with constant trituration, alternately, first a little of the oleo-resin, then a little water, and so on till the whole of the copaiba is emulsified; place in a bottle, add more water, and, lastly, the spt. aeth. nit. By this procedure the copaiba is minutely divided, and each particle coated with mucilage. If the mucilage were added to the copaiba, or even the whole of the copaiba added at once to the mucilage, a different result would be effected. The copaiba would doubtless be broken up, but instead of being coated by, it would form the covering of, particles of mucilage, and on standing would quickly separate.

Mix the liq. potassae with 3 drachms of water, add the oil, shake until thoroughly emulsified, add more water, and the vin. ipecac.last.

Our next example is an examination prescription upon which a candidate failed:

Rub the oil down with 28 grains of pulv. acaciae (which is the equivalent of 3j. of mucilage), and when that is thoroughly mixed add all at once 3j. of water, after which triturate until the whole is emulsified ; gradually add more water in small quantities; lastly add the syrup and tinct. zingib., both previously diluted with a little water. Made according to these directions a uniform preparation is produced which does not separate until kept for some time. The prescription is not a difficult one, but requires careful manipulation. In making emulsions it should be borne in mind that syrups, and preparations containing spirit, should invariably be added last.

The question as to whether emulsions of oils and oleo-resins should be made with mortar and pestle or in a bottle has been often discussed, and need not be entered on here, it now being almost universally agreed that the mortar and pestle are best, and a broad pestle should always be used.