An emulsion is a mixture of small particles of solid or liquid substances in a liquid vehicle, the particles being kept in suspension, or easily diffusible, and prevented from coalescing by means of a body called an emulsifier. Many examples of such mixtures occur at the dispensing-counter, but the term ' emulsion' is generally reserved for fluid aqueous mixtures of oils and oleo-resins. In the widest sense it includes any liquid preparations containing something in suspension but not dissolved and not easily separable. For example, a mixture of tr. quininae ammon. in water, with the quinine hydrate kept from adhering to the sides of the bottle by means of mucilage, is, strictly speaking, an emulsion, but is not generally so regarded by dispensers. Hence, such mixtures are not included in this chapter; but we ought properly to regard as emulsions all.