I EXPANSION

in physics, the inerease of the dimensions of a body caused by a change of temperature. In general, bodies expand with rise of temperature, solids expanding least and gases most. This change in the dimensions of solids with temperature is expansion in length, area, and volume. These changes are referred to a unit called the coetlicient of expansion, or expansion of unit length, area, or volume for 1° rise. For example, the coefficient of linear expansion of iron is 000011; that is, a yard, sav, of iron rod becomes longer by this fraction of a yard for each degree cent i Section of Exogenous Plant grade that its temperature is raised. The coefficient of volume expansion of a solid is three times its linear coefficient. Crystals have different rates of expansion along the three crystalline axes. Fused silica or quartz expands so slightly that it may be plunged when red-hot into water without being cracked. Invar, a nickel steel alloy, which also has an extremely small coefficient of expansion, is used in making clock pendulums, which are unaffected by change of temperature. The observed expansion ot liquids is affected by the expansion of the ve el containing them, and the apparent expansion of a liquid is thus always le than its real expansion. The expansion of water with rise of temperature is irregular; water contracts from 0° to 4° C., and thereafter expands at an increasing rate until the boiling-point is reached. The expansion of mercury and other liquids is employed in thermometry. Gases when kept at constant pre ure expand by about stqrd part of their volume at 0° C. for each degree rise. 359