A material composed of gas bubbles separated from one another by liquid or solid sheets (or lamellae). Foam densities range from nearly zero to about 43 lb/ft³ (0.7 g/cm³), above which gas emulsions rather than foams are formed. The bubbles are spherical when the lamellae that separate the bubbles are thick (approximately 0.04 in. or 1 mm); the bubbles are polyhedral when the lamellae are thin (approximately 0.0004 in. or 0.01 mm). Pure liquids do not foam; that is, they cannot produce liquid lamellae of any permanence. Relatively permanent lamellae are created only when a substance is present that is adsorbed at the surface of the liquid. Such substances may be in true solution in the liquid, or they may be particles of a finely divided solid which, because of poor wetting by the liquid, remain at the surface. In both cases, surface layers rich in the added substance are formed. The inability of the adsorbed substance to enter the bulk of the liquid preserves the surface and so confers stability on the foam lamellae. However, this stability is transient. Gravity drains solution out of the lamellae, and the solubility of gas in the solution, although slight, allows gas to diffuse out of the foam. These destabilising factors tend to separate the liquid and the gas phases of the foam and so destroy it.