The Phase Rule and Its Applications - Alexander Findlay

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the velocity with which the transformation of the tin occurs is increased, and Cohen and van Eyk found that the temperature of maximum velocity is about -50°. Contact with the stable form will, of course, facilitate the transformation.

      The change of white tin into grey takes place also with increased velocity in presence of a solution of tin ammonium chloride (pink salt), which is able to dissolve small quantities of tin. In presence of such a solution also, it was found that the temperature at which the velocity of transformation was greatest was raised to 0°. At this temperature, white tin in contact with a solution of tin ammonium chloride, and the grey modification, undergoes transformation to an appreciable extent in the course of a few days.

      Fig. 7 is a photograph of a piece of white tin undergoing transformation into the grey variety.[62] The bright surface of the tin becomes covered with a number of warty masses, formed of the less dense grey form, and the number and size of these continue to grow until the whole of the white tin has passed into a grey powder. On account of the appearance which is here seen, this transformation of tin has been called by Cohen the "tin plague."

      

Fig. 7.

      

      Enantiotropy and Monotropy.—In the case of sulphur and tin, we have met with two substances existing in polymorphic forms, and we have also learned that these forms exhibit a definite transition point at which their relative stability is reversed. Each form, therefore, possesses a definite range of stable existence, and is capable of undergoing transformation into the other, at temperatures above or below that of the transition point.

      Another class of dimorphous substances is, however, met with as, for instance, in the case of the well-known compounds iodine monochloride and benzophenone. Each crystalline form has its own melting point, the dimorphous forms of iodine monochloride melting at 13.9° and 27.2°,[63] and those of benzophenone at 26° and 48°.[64] This class of substance differs from that which we have already studied (e.g. sulphur and tin), in that at all temperatures up to the melting point, only one of the forms is stable, the other being metastable. There is, therefore, no transition point, and transformation of the crystalline forms can be observed only in one direction. These two classes of phenomena are distinguished by the names enantiotropy and monotropy; enantiotropic substances being such that the change of one form into the other is a reversible process (e.g. rhombic sulphur into monoclinic, and monoclinic sulphur into rhombic), and monotropic substances, those in which the transformation of the crystalline forms is irreversible.

      

Fig. 9.

      

Fig. 8.

      These differences in the behaviour can be explained very well in many cases by supposing that in the case of enantiotropic substances the transition point lies below the melting point, while in the case of monotropic substances, it lies above the melting point.[65] These conditions would be represented by the Figs. 8 and 9.

      In these two figures, O3 is the transition point, O1 and O2 the melting points of the metastable and stable forms respectively. From Fig. 9 we see that the crystalline form I. at all temperatures up to its melting point is metastable with respect to the form II. In such cases the transition point could be reached only at higher pressures.

      Although, as already stated, this explanation suffices for many cases, it does not prove that in all cases of monotropy the transition point is above the melting point of the two forms. It is also quite possible that the transition point may lie below the melting points;[66] in this case we have what is known as pseudomonotropy. It is possible that graphite and diamond,[67] perhaps also the two forms of phosphorus, stand in the relation of pseudomonotropy (v. p. 49).

      The disposition of the curves in Figs. 8 and 9 also explains the phenomenon sometimes met with, especially in organic chemistry, that the substance first melts, then solidifies, and remelts at a higher temperature. On again determining the melting point after re-solidification, only the higher melting point is obtained.

      D. Phosphorus.

      An interesting case of a monotropic dimorphous substance is found in phosphorus, which occurs in two crystalline forms; white phosphorus belonging to the regular system, and red phosphorus belonging to the hexagonal system. From determinations of the vapour pressures of liquid white phosphorus, and of solid red phosphorus,[68] it was found that the vapour pressure of red phosphorus was considerably lower than that of liquid white phosphorus at the same temperature, the values obtained being given in the following table.

      Vapour Pressures of White and Red Phosphorus.

Vapour pressure of liquid white phosphorus. Vapour pressure of red phosphorus.
Temperature. Pressure in cm. Temperature. Pressure in atm. Temperature. Pressure in atm.
165° 12 360° 3.2 360° 0.1
180° 20.4 440° 7.5 440° 1.75
200° 26.6 494° 18.0 487° 6.8
219° 35.9 503° 21.9 510° 10.8
230° 51.4 511° 26.2 531° 16.0
290° 76.0 — — 550° 31.0
— — — — 577° 56.0

      These values are also represented graphically in Fig. 10.

      

Fig. 10.

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