Page:The New International Encyclopædia 1st ed. v. 13.djvu/332

* MELTING-POINT. 298 MELTJSINA. Instead, the investigator uses much larger quan- tities of substance, reduces the hitter to a fine powder, and immerses the thermometer directly into it. While it is known that different niodi- fieations of one and the same chemical sub- stance may have ditTerent melting-points, and hence the melting-point caimot be considered as strictlj' characteristic of a given chemical spe- cies, it is so easy to determine with great pre- cision, that it is considered as one of the most useful constants and is very frequently em- ployed by chemists for the purpose of identify- ing substances, and as already mentioned for the purpose of testing tlieir purity. Further, in spite of but too many exceptions, certain interesting relations liave been shown beyond doubt to exist between the melting-points of organic substances and their molecular weights and constitution. See article Boilixg-Poixt. Under Feeezing-Poixt that point has been defined with reference to the vaijor-tension of the given substance in tlie solid and liquid states. The same definition, and for precisely the same reasons, is of course applicable to the melt- ing-point. For the "latent heat of fusion,' see Fbeezixg MiXTi kes and Heat. Influence of External Pressuke ox' Melt- iNG-PoiNTS. Strictly speaking, the melting-point of a solid substance, just as the boiling-point (q.v.) of a liquid, depends upon the external pressure. In the case of the melting-point, how- ever, the influence exercised by the external pressure is so very slight that it may generally be safely left out of account altogether. The subject was first theoretically investigated, from the standpoint of thermodynamics, by .lames Thomson, who found that for a given substance the change of melting-temperature caused by an increase of one atmosphere in pressure must be represented by the formula, T(V — V) r where T denotes the melting-point (on the abso- lute scale, i.e. the centigrade temperature in- creased by 27.3) corresponding to some given pres- sure; V denotes the volume occupied by one gram of the liquid substance at the melting-tem- perature; V denotes the volume occupied by the solid substance at the same temjierature: and r denotes the mechanical equivalent ( in terms of Miter-atmospheres') of the heal absorlied, at the same temperature, during the melting of one gram of the substance. (By a 'liter-atmospliere' is meant the minimum mechanical work required to cause a dimiiuition of <me liter in any volume, against the constant resistance of a pressure of one atmosphere.) Tt will be observed that if V is greater th.nn V, i.e. if the melting is accom- panied by an increase in volume, the above expres- sion is positive, and hence an increase of pres- sure causes the melting-temiierature to rise. On the contrary, if V' is greater than V, the ex- pression is negative, and hence nn increase of pressure causes the melting-temperature to fall. These theoretical results are in perfect agree- ment with experimental obsen-ation. In the case of iee. melting is accompanied by a eontrnction in volume, i.e, V is greater than V, and the above formula leads to the result that while im- der normal atmospheric pressure ice melts at 0° C. (32° F.), the melting-point under a pres- sure of two atmospheres would be — 0.0077° C. (31.9801° F.). As far back as 1851 William Thomson (Lord Kelvin) obtained practically the same result by direct observation, and the for- mula has since been found to hold similarly good in the ease of all substances examined, EiTECTic MiXTiKES. It Was shown in the article Freezixg-Poixt that if a solution happens to be saturated at its freezing-point, the solid gradually separating out by freezing and the liquid remaining unfrozen cannot but have the same composition, and hence the freezing-tem- perature must remain constant. It is perfectly obvious that the solid mixture thus obtained nmst entirely melt at the same constant tem- perature. ^Mixtures of salts thus obtained from solutions are termed "cryohydrates.' More gen- erally, and whatever its origin, a mechanical mix- ture that melts at a constant tem|)eratuie lower than that of any other mixture of the same sub- stances, is termed a 'euteetic mixture.' Follow- ing are a few examjiles of euteetic alloys: an alloy containing 55.58 per cent, of bismuth and 44,42 per cent, of lead melts at the constant tem- perature of 122.7° C, (252.80° F.) ; an alloy of 40.70 per cent, of bismuth and 53.30 per cent, of tin melts at 133° C. (271.4° F.) : an alloy of 59.19 per cent, of bismuth and 40.81 per cent, of cadmium melts at 144° C. (291.2° F.) ; an alloy of 92.85 per cent, of bisnuith and 7.15 per cent. of zinc melts at 248° C. (478.4° F.) ; an alloy of 47.75 per cent, of bismuth, 18.30 per cent, lead, 13,31 per cent, of cadmium, and 20.00 per cent, of tin melts at 71° C. (159.8° F.). See Freeziso- Point; Fisible Metals. MELTON-MOWBRAY, mel'ton mo'brft. A market-town in Leicestershire. England. 10 miles northeast of Leicester, on the Eye, near its junc- tion with the Wreak (llap: England, F 4), Stil- ton cheese and pork, pies are extensively made for the London, Manchester, and Leeds markets. It has large quarries of iron ore. and smelting fir- naces, and an important cattle market. It is the Midlands "metroiiolis of fox-hunting.' with numer- ous hunting seats, and in the winter is crowded with sportsmen. It is a very ancient town and in 1044 was the scene of a defeat and slaughter of the Parliamentarians. Population, in 1891, 03(12: ill 1901. 7.500. MELUN, me-lex'. An ancient town of France; the capital of the Department of Seine- et-M:une; built on an island and on both banks of the Seine, 28 miles southeast of Paris (Map: France, J 3). The town is an important rail- road centre. The manufactures are carriages, cot ton and woolen goods, earthenware, and chiK'n late; and there is trade in yrain, flour, cattle. and fowls. Population, in 1901, 13.059. ^Iclun was taken five times during the ninth century l>y the Northmen, It fell into the hand< of the l^n:;- lish in 1419, and was held by them for ten viMr-. MELUSITJA. A powerful water fairy, half woman and half fish, in French folklore. She l)ecame the wife of Count Raymond of Poitier- nnd built a castle which from her own name wn- calliMi I.u-^inia, the present Lusignan. When hei husband surprised her in her double form in th- bath she disappeared. When the death of a member of the family or of the Kins '^f France was about to occur sbe appeared in a high tower of the castle, in mourning garments, and an- nounced the approaching event by three shrill cries. The legend was used in 1387 by Jean