84 



NA TURE 



[November i8, 1909 



The case of sulphur is one of great interest. It has 

 long been known that sulphur can exist in at least three 

 solid forms. It crystallises from some solvents in octa- 

 hedral crystals, from others or from its liquid state in 

 monoclinic crystals. In the latter case some amorphous 

 sulphur is generally dissolved in the crystals, and the 

 amorphous variety itself is formed in tough vitreous 

 masses when molten sulphur, heated until it becomes very 

 viscous, is poured into cold water. At ordinary tempera- 

 tures the octahedral form alone is stable. It has been 

 found that at atmospheric pressure octahedral sulphur is 

 converted into monoclinic at 954° C, and in the process 

 2.7 gram-calories per gram of sulphur are evolved. The 

 density of octahedral sulphur is about 2 03 and of mono- 

 clinic about 1-98 at ordinary temperatures. In accordance 

 with the principles developed previously, the transforma- 

 tion temperature of rhombic to monoclinic sulphur must 

 rise with increase of pressure. So far back as 1S87 



Rhombic 



Liquid 



Vapour 



Temperature 



Roozeboom ' was able to predict that the diagram of con- 

 dition for sulphur would be as shown in Fig. 3. 



Prof. Tammann has supplied the corroboration of the 

 existence of the triple point. 



Suppose that we have sulphur at a pressure of about 

 1500 kg./sq. cm. (952 tons/sq. inch) and raise its tempera- 

 ture to about 160° C. or more, we shall cut the melting- 

 point curve of octahedral sulphur, and the sulphur will 

 melt. If we then allow the sulphur to cool, keeping the 

 pressure up, octahedral sulphur will crystallise from the 

 melt instead of monoclinic sulphur. This very likely has 

 some bearing on the occurrence of native crystals of octa- 

 hedral sulphur. 



i-; believed to be the octahedral system. This slide is then placed in the 

 projection microscope, when it is seen that its appearance is totally ditTerent 

 from that of the first slide. The preparation of octahedral sulphur is then 

 heated on the hot stage, and when the transform:ition temperature is reached 

 it is seen that the structure begins to change — the crystallisation breaks up 

 and becomes granular, the granules showing in general much more colour 

 than the original crystallisation. These granules are taken to be monoclinic 

 sulphur. The lemperature is now raised until about half the preparation has 

 melted, and it is then allowed 10 cool back a little so as to crystallise. The 

 crystals now show the characteristic monoclinic crystallisation with brilliant 

 colours, since unnielted monoclinic sulphur is present. 

 1 Rec. Trav. Chim. Pays-Bas, vi., 1887, 314. 



NO. 2090, VOL. 82] 



It is not every substance which has such sharply defined 

 properties as sulphur, though even these are not so sharp- 

 as they might be, owing to the constant presence of 

 amorphous sulphur. An instructive case is afforded by 

 phenol. As the diagram shows, there is a considerable 

 region of the field in which two kinds of crystals of 

 diiferent density can exist together, the curves forming the 

 boundary of this region of pseudo-equilibrium. 



It may be that the two crystalline forms of carbon which 

 apparently can exist together indefinitely at ordinary 

 temperatures and pressures are an illustration of the same 

 property. 



As a final illustration we may note the results for water 

 down to —80° C, from which it appears that it possesses^ 

 three allotropic crystalline forms with at least two melting 

 points. 



The melting curves of from thirty to forty substances 

 have been investigated, mainly by Tammann, up to about 

 3000 kg./sq. cm. = 1905 tons/sq. inch, and the general 

 result has been to show that there is a tendency for the 

 rate of change of melting temperature with pressure to 

 fall off as the temperature rises, and also that many sub- 

 stances, which at ordinary pressures crystallise in one 

 form only, can be caused to assume allotropic modifications 

 under high pressure. This tendency to form allotropic 

 modifications appears to be associated with the extent to 

 which a substance can be under-cooled without crystal- 

 lising. 



A question of the greatest interest and importance may 

 now be formulated. What will happen if w^e go on increas- 

 ing the pressure? Will a state of affairs be reached in 

 which it is no longer possible to distinguish between the 

 liquid and its crystalline form? Will there be, in fact, a 

 sort of critical point at which the melting curve will endr 

 At present we can only say that no indications of such an 

 occurrence have been observed experimentally, and Prof. 

 Tammann takes the point that it is highly improbable that 

 anything in the nature of continuous transformation can 

 take place, because a crj'stal has different properties in 

 different directions related to its axes, and there is thus 

 a much greater qualitative difference between crystals and 

 liquids than between liquids and gases, both of which are 

 isotropic. I must admit that this argument does not appeal 

 to me very strongly. If it be possible to compress a sub- 

 stance until it reaches a state in which, at one and the 

 same temperature, the liquid has the saine density as the 

 crystals, presumably the mean distance of the molecules 

 will be the same in both cases. I see nothing monstrous, 

 in the view that in these circumstances crystallisation may 

 set in gradually, and that it may not be possible to say 

 exactly when the liquid ceases to be a .fluid and becomes 

 a crystalline solid. There are no theo- 

 retical or other grounds for supposing 



that the phenomena of crystal growth, 



as observed when there is a change 

 of volume accompanying the crystal 

 forination, will necessarily hold when 

 no such change of volume occurs. 



If we refer to the theory of the change of m.p. by 

 pressure it is obvious that if either the change of volume 

 or the latent heat of melting vanish at any temperature 

 or pressure on the melting curve, then in the neighbour- 

 hood of this pressure the curve must degenerate to a point, 

 or small pressure changes will not affect the m.p. It 

 was pointed out, however, that there is a term or terms 

 depending on the square of the pressure, and if these were 

 relatively important the only thing we should notice would 

 be a change of curvature at the point under consideration. 

 It does not follow that there is no maximum or minimum 

 to the melting temperature of any particular substance 

 because the term in P" may be vanishingly small : it may 

 be (and generally is) of opposite sign to the term in P, 

 and in this case it is only a question of the relativi^ 

 iinportance of the terms where the maxiinum or minimunr 

 melting point lies. Damien's empirical formula expresses 

 piecisely the effect to which I refer. The practical result 

 which is of importance in questions affecting the condi- 

 tion of the inner layers of the earth is that we are not 

 entitled — in fact, it is wrong — to suppose that pressitre 

 must necessarily go on raising the melting point in- 

 definitely ; everything depends on the substance under con- 



