Fixed oils, such as almond, poppy, olive, linseed, and castor, and balsams, such as copaiba and Peru, are easily combined with water by means of emulsifiers, of which gum acacia is the best. Yolk of egg and tragacanth are also emulsifiers. An emulsion with oil cannot be made satisfactorily with less gum acacia than one-fourth the weight of the oil. The proportion of water should be at least half of the total weight of the oil and gum.

The emulsifying power of 10 grammes of gum acacia is estimated as equal to the yolk of a large hen's egg, to 1.25 gramme of tragacanth, or 1 gramme of salep.

These emulsions may be made in either of three ways :

(1) By adding the water to the powdered gum in a large mixture-mortar, mixing with a large-knobbed pestle ; then adding the oil or balsam all at once, and stirring till emulsified, which will require two or three minutes. This is generally the surest plan [and the so-called'Continental method.'- Ed.]. Or (2) the gum may be put in the mortar, the oil poured on the top of it, the water round it, and then all the substances quickly stirred together. This is the method mostly adopted. Or (3) the oil may be rubbed with the gum, and the water added gradually. The proportions for a good emulsion are:

The first method of emulsification described is not quite so speedy, but it is very sure. Sometimes castor oil or balsam of copaiba cannot be emulsified by the second or third process. Probably this happens in consequence of some peculiarity in the oil or balsam, and the fact that in the two latter cases the gum is not dissolved quickly enough to effect the emulsification.

[The third plan is one which is now much followed in this country and recommended by leading pharmacists. Expert-ness is, however, required in order to have successful results. The probability is that moderately good results may be obtained by any one of the processes if the dispenser is in the habit of following it. If he gets good results from any particular process, we should not advise any experiments in the way of changing it. An emulsion at the best is a ticklish affair, and we have generally found it better to let well alone.- Ed.]

Salts, extracts, or other solid bodies to be dissolved in emulsions should be separately dissolved in some of the water to be added, and mixed with the emulsion. If mixed with the emulsifier a separation of some of the oil will occur.

Lycopodium, which is often ordered with oil emulsions, causes such a separation with remarkable rapidity. It is best to emulsify the oil and the lycopodium separately, and to mix the two in diluted form.

Borax added to a gum-acacia emulsion converts the latter into a jelly. Diluted acetic acid restores the fluidity, but such an addition is by no means justifiable. If an emulsion with borax and gum be ordered in a prescription, the mixture must be sent out in a wide-mouthed bottle. The gelatinisation does not occur till several hours after the mixing.

Balsam of Peru to be combined with an oil emulsion should be mixed with two-thirds of its weight of water, and stirred in with the emulsion in a mortar.

[The better plan is to mix about \ part of 90-per-cent. alcohol with 1 part of glycerin, and to this add the balsam of Peru, then incorporate thoroughly. This may be diluted to a proper miscible point for an emulsion by the further addition of water, plain, or, better still, a mixture of glycerin and water gradually added.- Ed.]

Emulsions with tragacanth do not keep well, and are but seldom ordered. For their preparation 1 gramme of tragacanth is mixed with 20 grammes of water. To this 20 grammes of oil and 10 grammes of water are added at once, and combined into an emulsion with constant stirring. The rest of the water is added gradually.

[The statement about the keeping properties of tragacanth emulsions is a mistake, as is testified by the sweetness of cod-liver-oil emulsions prepared with tragacanth. Tragacanth has properties as an emulsifying agent which in some respects are unequalled for such an oil as cod-liver-oil.- Ed.]

If spirit of wine, concentrated acids, solutions of salts, or tannin substances are to be mixed with emulsions, they should always be added in as diluted a form as possible.