MELTING POINT, the temperature at which the solid and liquid states of a body can coexist, without the fluid part of the mixture solidifying, nor the solid part melting. The melting point of a body which is crystalline in nature (like ice) is usually quite definite, and a body of this kind, when it melts, passes at once from a state of perfect solidity into a state of perfect fluidity. The melting point of an amorphous body, such as wax or pitch, is often very indefinite; and when a solid of this sort is gradually heated it grows soft and passes into the fluid state by a process which is practically (or even absolutely) continuous. The phenom ena of the fusion and solidification of amor phous bodies like wax are not yet understood as well as those that attend the change of state of a crystalline body; and it is to be understood that what follows in the present article is stated with particular reference to the crystal line class of substances, in which the melting point is definite.
In general, solid bodies expand upon melt ing, the original solid having a greater density than the liquid that results from its fusion. In the case of water, as is well known, the reverse is true; the fact that ice floats in water proving that the water is denser. The fact that the density of a substance changes upon fusion plies (as may be proved by the mechanical the ory of heat) that the melting point of the sub stance cannot be entirely independent of • the pressure.' This fact was in by James Thomson, whb showed 'that the. melting' point of a substance like ice, which upon melting, must he lowered pres,sure; while the melting point of one which expands upon melting must be raised by, pressure.. These theoretical conclusions have' since been abun dantly verified by experiment. In the case of ice, for example, it has been found by Dewar, that the melting point is lowered by 0.0130°.,Ki per atmosphere of ,increase of pxes,spre, tall. to 700 atmospheres.
When a substance passes from the solid to the liquid state, it absorb's a very, considerable amount of heat, which known as the "latent heat of liquefactioe ; and the liquid which is formed cannot be all reduced to the solid state again until an exactly equal heat has been abstracted from it. When heat' is added to a mixture composed of a solid and its liquid (pounded ice and water,, example), it does not increase the temperature of the mixture, but merely causes a certain amount of the solid to , melt. This continues so long as any of the un melted solid remains; each addition of heat merely bringing about a corresponding amount of liquefaction, while the temperature of the mixture remains tmaanged. It is only after all of the solid has melted that the further addition' of heat will produce elevation of temperature.
The melting points of compounds undoubtedly depend, in some manner, upon their molecular, constitutions, but we have, as yet, no definite knowledge of the nature of We know that in a homologOtis series of er6hic compounds, the melting points show a strong' tendency to increase, as the molecular weights ' of the members of the series ihCreaSe ; hut the' change in the melting ,points is irregular as we' proceed from member to 'member, and no has yet been discovered. which takes the changes fully into account. The melting point of a mix ture (such as an alloy) "is often lowenthan that of, any one of the constituents, and.the effect of. the presence of an impurity in a given substance upon the melting point of that substance is often out of all apparent proportion to the quantity of impurity present. This fact implies that in determination of a melting point it it highly im portant that the substance under examination shall be as nearly pure as possible. The pres-, ence of slight, unrecognized and apparently neg. ligible impurities has sometimes led to entirely erroneous conclusions. In 1860, for' example, Kolbe announced the discovery of an acid iso meric with benzoic acid, hut having a lower melting point; but Beilstein afterward proved that the substance in question was merely or dinary benzoic acid contaminated 'with a cum-, tity of chlorobenzoic acid so small as to be barely recognizable by analysis.
For determining the melting 'point of min erals, the instrument devised by Joly and known as the meldometer, is sometimes very con venient. It consists essentially of a strip of platinum foil, upon which small fragments of the minerals are laid. The is then' gradually heated by the passage of an electric current, until the mineral melts; the tempera ture of the foil being obtained by a calculation based 'upon the resistance of the platinum, the observed strength of the current and the rate at which heat is radiated from the foil, as de termined by separate observations. This method is especially useful for. tdinerals which can be had only in stnall fragments.
The term fusing point is often used instead of melting point, where a high temperature is respired 'to melt. Following are the melting or fusing points of a few familiar elements in Mercuinf; 38; ice, 32; potash. 104.5 • ,soda, 1 204; sulphur, 23,5; bismuth, 517; tin, 551; lead, 850; aluminum, 1,160; flint, ,600; sil ver, 1,?50; gold, 11,913; copper, 1,931; nickel, 2,642; iron, 2,912; platinum, 3,225; carbon, in fusible. Alloys show vastly different melting points; ,thus 93 per cent bismuth with 7 per cent zinc melted at 479° P.; 44 per cent lead and 56 per cent bismuth at See FREEZING Poner.