MELTING-POINT. The temperature at which a given substance passes from the solid into the liquid state. Different substances gen erally have different melting-points. Thus, mercury if solidified by cold would melt at a temperature of below 0° C. (-40° F.) : ice melts at 0° C. (32° F.) ; sulphur at 115' C. (239° F.) ; tin at 230' C. (446° F.) ; lead at 324° C. (615° F.) ; zinc at 418° C. (784° F.) ; aluminum at 727° C. (1341° F.) ; silver at 968° C'. (1774° F.) ; gold at 1072° C. (1862' F.) ; copper at 1052° C'. (1980° E.) ; pure iron at 1704° C. (3099° F.) ; platinum at 1777° C. (3231° F.) ; etc. The presence cif more or less impurity in a given substance generally causes a corresponding depression of its melt ing-point, and hence the latter is often de termined when it is required to ascertain whether a given sub stance, especially a carbon compound of known melting- point is perfectly pure. Such determinations may lie conveniently carried out by means of the apparatus shown in the accompanying fig The apparatus con sists of an ordinary round-bottomed flask of about 2.50 cubic centimeters capacity, with the greater part of the neck cut off: in this is an ordinary test-tube widened in one place so as to be readily held by the flask without touching its bottom; both contain, to about the same level, some liquid (say, strung sul phuric acid ) that may be heated to a somewhat high temperature without boiling. To carry out a determination. a small amount of the given sub stance is introduced into a capillary tube of glass, the latter is tied on to a thermometer so that the substance is very near the mercury bulb, the thermometer with the capillary tube is im mersed in the liquid of the test-tube, the tem perature is allowed to rise very slowly, and the point is carefully noted at which the contents of the capillary tube begin to change color and be come transparent. For very precise determina
tions. however, this method cannot be employed.
Instead, the investigator uses much larger quan tities of substance, reduces the latter to a fine powder, and immerses the thermometer directly into it. \\ hile it is known that different modi fications of one and the same chemical sub stance may have different nwlting-points, and hemp the melting-point cannot he considered as strictly 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 their purity. Further, in spite of but too many exceptions, certain interesting relations have been shown beyond doubt to exist between the melting-points of organic substances and their molecular weights and constitution. See article liontxn-PoINT.
Under FREEZING-POINT that point has been defined with reference to the vapor-tension of the given substance in the 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 FREEZING 'MIXTURES and HEAT.