LIQUIDITY is that condition of a material substance in which the partible. have a perfect freedom of motion, without any sensible ten dency to approach to or recede from one another, except by the action of some external power. Liquidity is therefore comprehended In the condition of fluidity, the latter term being applied as well to gases, and even to the principle of electricity, Inrignetiam, &c., as to water, oil, eke., which are properly called liquids. [Ficum ; FLUIDITY.] The phenomena of capillary or molecular action show that the attractions which constitute what are called the affinities of substances extend to very small distances only from the particles ; and hence, when the particles of a substance aro situated beyond the limits of such attractive forces, the repulsive power, arising probably from the action of caloric, causes the particles to recede continually from one another, and induces the state of aeriform fluidity. [Arrnsortox.] Now the phenomena of crystallisation seem to indicate that the attrac tion of affinity is exerted with greater or less intensity according as the like or unlike aides of the particles of a substance present themselves to one another in their mutual approaches; but it is probable that this modification of the attraction of affinity extends to less distances from the centres of gravity of the particles than the general attraction extends : hence, when the particles of a substance are, from any cause, brought so near one another that the attraction of affinity is in equl librio with the repulsive force of the calorie, and at the same time the modification of that attraction caused by the various positions which the particles assume in approaching one another, entirely or nearly vanishes, it should follow that the particles become freely moveable in any direction about one another, whatever be their form ; and thus may arise the condition of liquidity.
It may be added that, if the particles of a substance be, by the abstraction of caloric, made to approach still nearer to one another, the attraction of affinity will exceed the force of repulsion ; and there will be constituted a solid body, which may be crystallised or not according as the particles approach one another gradually or otherwise : in tho former case they may arrange themselves -in such positions as to become connected together in one direction by the sides at which the attraction is the greatest; while, if the approach is rapid and is accompanied by agitation, the union of the particles may take place irregularly.
The particles of a liquid are held together with considerable force notwithstanding their freedom of motion, since a small quantity of a liquid has a tendency to take a spherical form when at a distance front any substance for which its particles have greater affinity than for one another : this is very evident in mercury, oil, and water, the first of which on being suffered to fall on a table immediately divides itself into globules, and the others take a like form when a small quantity of either is suspended from a pointed extremity of any object.
The dilatations of water and mercury by the application of heat, as well as the remarkable fact that the expansions of water are equal at temperatures which are at equal distances above and below about 39° Fehr., will be noticed under THERMOMETER. See also HEAT. But the expansions of any liquid, at different temperatures, for equal increments of caloric, are not equal to one another ; and indeed the same remark may be extended to solids, as shown under HEAT. The phenomena of molecular action in liquids aro noticed under CAPILLARY ATTRACTION, DIFFUSION, &C. See also MOLECULAR ATTRACTION, and for the specific heat of liquids, or their capacity for caloric, SPECIFIC HEAT.