Acacia Gum is the principal gum employed by pharmacists. It is typical of a large class of bodies which emulsify oils, but few of them equal it in convenience and power. The gum contains the calcium, magnesium, and potassium salts of arabic acid, but there are many varieties of the gum, and they differ in composition and emulsifying power. From the nature of the emulsions of oils formed by acacia it is safe to assume that the oil exists in the emulsion in a similar condition to the fat in milk- that is, the particles are coated with a film of gum (probably in molecular association with the oil); this prevents coalescing, and the viscosity produced in the mixture prevents separation. The fact that the acacia gums which are the poorest emulsifiers generally give the most viscous mucilages indicates that the filming process has something to do with perfection of emulsification, these poor gums being less rich in the arabates than gums answering the official description. The so-called ' English method' of using gum acacia in emulsifying oils is to triturate 1 part of the powdered gum in a mortar with 2 parts of oil, add at one time 1 1/2 part of water, and triturate, when, after a few turns of the pestle, the whole is emulsified. The rest of the water may be added by degrees.

At least 3 parts of the powdered gum is required for 8 parts of an essential oil, and 2 parts for 8 parts of fixed oils and balsams, but double these quantities is generally required, according to the amount of water which is present. Why the dry-gum method should be called ' English' by American writers we do not know, for the other, or 'continental,' way (as exemplified by mistura olei ricini) is as commonly followed. Powdered acacia of commerce is not well suited to the preparation of emulsions. It usually contains dust particles, and frequently in sufficient quantity to give the emulsion a grey appearance. Besides, it is nearly always very acid. The best procedure is to powder picked gum extemporaneously; or, if the operator prefer to work with mucilage, this can be prepared quickly from gum well ground down and triturated with the water. For manipulative details see page 337. Some acacias yield a mucilage affording a strong acid reaction. The amount of acidity, however, is very small, and to such mucilages it is permissible to add just sufficient liq. calcis sacch. to bring the acidity down until it very faintly changes the colour of blue litmus-paper, but not enough to produce alkalinity.

As a point of commercial interest it may be noted that unpowdered acacia is much less expensive than commercial powder of equivalent quality.

Tragacanth Gum is much inferior to acacia gum as an emulsifier. It does not assist the compounder in dividing the oil into small particles, as acacia gum does, and its action is solely that of a viscoid body- it keeps the oil-globules apart once they are formed. Its influence is well exemplified in the cod-liver-oil emulsion which is illustrated in d, page 314. Tragacanth is capable of doing much better than this when powerful mechanical force is employed, but it is scarcely possible to do better at the dispensing-counter. A judicious use of acacia and tragacanth together produces the most serviceable emulsion. Emulsions prepared with acacia always separate unless an undesirable quantity of the gum has been used. Acacia breaks up oil beautifully, but, owing to lack of viscosity, it does not impart sufficient ' body' to the liquid to prevent separation. Tragacanth supplies the necessary viscosity : two or three grains to the ounce suffices, depending on the quantity of acacia used.

Starch is in some respects like tragacanth ; that is to say, it is as good a suspending agent, but lacks the slightly gummy property of tragacanth. It is employed (as starch mucilage) for making iodoform emulsion, and for suspending or emulsifying medicines which are given by the rectum.

Yolk of Egg is a remarkable emulsifying agent. Sometimes the whole egg is used without disadvantage, if there is nothing in the mixture to coagulate the albumen. The yolk is itself an emulsion of 30 per cent of fat with 16 per cent, of vitellin, a substance closely resembling the casein of milk in composition and properties. There are also a little albumen and various earthy and alkaline salts in egg-yolk. The vitellin may be regarded as the substantial emulsifying ingredient. It is particularly serviceable for limpid oils, such as turpentine, and it forms emulsions very quickly, perhaps because the emulsifier is a potential emulsion.

Milk is one of the best emulsifying agents for extract of male fern, and, strange to say, fresh milk does better than the skimmed variety, thus showing that emulsification is not entirely due to casein.

Casein prepared from milk is now obtainable, and is an excellent emulsifier. The casein may be made as follows : Take 1 gallon of milk and 2 1/2 ounces of solution of ammonia, and after shaking well set the mixture aside for twenty-four hours. Two layers are now observed, the semi-saponified butter above and the lacto-serum below. The lower liquid is drawn off, and casein is precipitated from it by acetic acid. The magma is collected and strongly pressed to expel moisture 2 1/2 drachms of sodium bicarbonate is now added by trituration, and, finally, enough sugar to obtain a powder representing when dry about 10 per cent. of its weight of casein. The preparation keeps well in securely corked bottles for at least three years. It has a slight, not unpleasant, smell, which is not appreciable in preparations. M. Leger, who suggested the use of casein in 1887, divides substances into (1) those soluble in alcohol, and (2) those insoluble. The first class, which includes resins, balsams, and oleo-resins, can be emulsified in a bottle. The product is first weighed or measured into the bottle, and enough alcohol is added to dissolve it.

For a 4-ounce mixture use about 2 1/2 drachms of saccharated casein, dissolved in an equal weight of water, add the solution, thoroughly shake, and make up to the required volume with the rest of the water gradually introduced, with continual shaking. For oils a mortar is used. A piece of cheese- Gruyere or white Cheddar - is a splendid emulsifier : about an ounce to an 8-ounce emulsion. The oil should be triturated with the cheese, which to some extent dissolves in the oil, then the other liquids are incorporated. It is suitable only for emulsions to be consumed within a few days after it has been made. Like acacia it requires the aid of a viscous substance, such as tragacanth or Irish moss, to form a non-separating emulsion.