INFLUENCE OF EXTERNAL PRESSURE ON 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 James 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 y where '1' denotes the melting-point (on the abso lute scale, i.e. the centigrade temperature in creased by 273) corresponding to some given pres sure; V denotes the volume occupied by one grain of the liquid substance at the melting-tem perature; &nides the volume occupied by the solid substance at the same temperature: and r denotes the mechanical equivalent (in terms of 'lifer-atmospheres') of the heat absorbed, at the same temperature, during the melting of one gram of the substance. (fly a 'liter-atmosphere' is meant the minimum mechanical work required to cause a diminution of incI liter in any volume.
against the constant resistance of a pressure of one t mosphe It will be observed that if V is greater than V'. i.e. if the melting is ;teflon partied by an increase in volume, the above expres sion is positive. and hence an increase of pres sure causes the melting-temperature to rise. On the contrary. if V' is greater than V. the ex pression is negative. and hence an itterea:,-. of pressure causes the melting-temperature to fall. These theoretieal results are in perfect agree ment with experimental observation. In the case of ice, melting is accmnpanied by a contraction in volume. i.e. V' is greater than V. and the alxn•e formula leads to the result that while un der normal atinospherie pressure ice melts at 0° C. (32° VI. the melting-point under a pres sure of two atmospheres would be -0.0077° C.
(31.9861° 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 case of all substances examined.