2. (Physics) The ratio of mass, or quantity of matter, to
bulk or volume, esp. as compared with the mass and volume
of a portion of some substance used as a standard.
Note: For gases the standard substance is hydrogen, at a
temperature of 0[deg] Centigrade and a pressure of 760
millimeters. For liquids and solids the standard is
water at a temperature of 4[deg] Centigrade. The
density of solids and liquids is usually called
specific gravity, and the same is true of gases when
referred to air as a standard.
3. (Photog.) Depth of shade. --Abney.