EXPANSION, in natural philosophy, he enlargement or increase of bulk in dies, chiefly by means of heat. This s one of the most general effects of ca oric, being common to all bodies what ver, whether solid or fluid, or in an aeri orm state. In some cases bodies seem o expand as they grow cold, as water in e act of freezing ; this, however, is nown to be no exception to the general rule, but is owing to the arrangement of he particles, or to crystallization, and is not a regular and gradual expansion, like hat of metals, or other solid substances, y means of heat. In various metals, kewise, an expansion takes place in assing from a fluid to a solid state, which is accounted for in the same way. The expansion of solids is exhibited by the PYROMETRIL (which see.) A rod of iron, for instance, becomes sensibly long er and larger in all its dimensions in pass ing from a low to a high state of tempe rature. The expansion of fluids is shewn
by the thermometer, and is the princi ple upon which that useful instrument is constructed ; by immersing a thermome ter into hot water, the mercury, or other fluid, contained in it, immediately ex pands. See THERMOMETER. The degree of expansion produced in different li quids varies very considerably. In ge neral, the denser the fluid, the less the expansion ; water expands more than mercury, and alcohol, which is lighter than water, expands more than water. The expansion of aeriform fluids may be exhibited by bringing a bladder, partly filled with air, and the neck closely tied, near the fire ; the bladder will soon be distended, and will, if the heat be strong enough, burst. Metals expand in the following order; those that expand most are placed first : zinc, lead, tin, copper, bismuth, iron, platina.