Milk is the ideal emulsion. It is a mixture of almost equal small percentages of fat and emulsifying agents with sugar and water; the fat separates on standing, but is easily distributed again, and the fat particles coalesce (de-emulsify) with great difficulty. Cow's milk contains on the average 3.6 per cent of fat, 3.3 per cent, of proteids (casein and albumin), 4/8 per cent, of milk sugar, 073 per cent, of inorganic salts, and the rest water. Casein is the emulsifying agent, but the albumin plays its part in this respect, and it seems as if the fat particles were covered by an extremely thin skin of proteid, whereby de-emulsification is prevented.

The emulsions which chemists generally have to prepare are much richer in fat than milk is, but compounders have to keep the ideal in view, for a perfect emulsion is one in which the oil-globules are invisible to the naked eye. Something short of this may look well, but it is only when the more perfect form is attained that it can safely and properly fulfil all its requirements in dispensing.

A.

B.

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D.

The figures given here illustrate the various degrees of perfection and badness of emulsification. a shows the appearance of milk under the microscope; b is a cod-liver-oil emulsion made with yolk of egg; c is a cod-liver-oil emulsion made with acacia ; and d one made with tragacanth. These are magnified about 100 diameters.

Several B.P. preparations are emulsions, e.g. :

Linimenta Ammonia, Calcis, et Hydrargyria in which some alkali forms soap with an oil, and the resulting soap emulsifies the rest of the oil.

Mistura Ammoniaci

Ammoniacum is a gum-resin containing 2 per cent, of essential oil as well as 28 per cent of gum and the rest resin. There is sufficient gum in it to keep the resin and oil in suspension when the whole is rubbed with water to a thin paste and further diluted. This may be called a 'natural emulsion.'

Mistura Amygdalae - If sweet almonds are skinned, beaten to a paste, and water added, a white emulsion is formed, which, however, soon separates oil, because there is less than I part of emulsifier (gum) in the almonds for every 5 parts of oil. The B.P. therefore adds gum acacia and sugar to the almonds (pulv. amygdalae co.), and this mixture forms with water a good emulsion.

Mistura Guaiaci

Here the artificial emulsifiers gum tragacanth and sugar are added to keep the particles of guaiacum resin from coalescing.

Mistura Olei Ricini

In a litre mixture 100 grammes of powdered acacia is employed to emulsify 375 millilitres of castor oil, sufficient orange-flower and cinnamon waters being used to complete the emulsion.

Mistura Scammonii (B.P. 1885). - A mixture of scammony (gum-resin) and fresh milk. There is a fair proportion of gum in scammony (about 20 per cent.), but insufficient to emulsify it with water, and the milk supplements its influence.