Fusing Point of a Solid

The method to be adopted in order to determine the fusing point of a solid must depend on several considerations, as -

(1) Whether the temperature can be registered on a mercury thermometer; i.e. does it lie between -40°C. and + 350°C.?

(2) Does the solid pass directly from the solid to the liquid state, or is there an intermediate viscous condition? If so, the melting point may be taken as somewhere between the temperature of the liquid and solid condition, but cannot be considered as a definite temperature.

(3) Whether or not the substance is a good conductor of heat If it be, the temperature of a vessel containing the substance in part solid will be very nearly constant if kept properly stirred. This is the case with ice and the fusible metals and alloys. For bodies which are bad conductors a method has to be adopted as occasion requires. We give as an instance the following, which is available in the case of paraffin wax.

The thermometer, when dipped into the melted paraffin, is wetted by the liquid, and when taken out is in consequence covered with a very thin and perfectly transparent film of liquid paraffin. This film cools, and on solidifying assumes a frosted appearance which extends rapidly all over the part of the thermometer that has been immersed. If the bulb of the thermometer is sufficiently small for us to neglect the difference of temperature between the interior and exterior portions of the mercury, the observation of the thermometer at the instant when this frosted appearance comes over the bulb may be taken as the melting point of paraffin. The only error likely to be introduced is that mentioned above, viz. that the temperature of the paraffin is not the mean temperature of the thermometer bulb. This can be rendered smaller and smaller by taking the liquid at temperatures approaching more and more nearly to the melting point as thus determined, and its direction can be reversed if we allow the paraffin to solidify on the bulb and then heat the bulb in a beaker of water and note the temperature at the instant when the film becomes transparent The mean of this temperature and that deduced from the previous experiment will be the melting point.