1909] on Experiments at High Temperatiires and Pressures. 547 



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 crys- 

 tallises from some solvents in octahedral 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 till it becomes very viscous is poured into cold 

 water. At ordinary temperatures the octahedral form alone is stable. 

 It has been found that at atmospheric pressure octahedral sulphur is 

 converted into monoclinic at 95 • 4° C. and in the process 2 • 7 gramme- 

 calories per gramme of sulphur are evolved. The density of octahe- 

 dral sulphur is about 2*0:') and of monoclinic about 1*98 at ordinary 

 temperatures. In accordance with the principles developed previously 

 the transformation temperature of rhombic to monoclinic sulphur 

 must rise with increase of pressure. So far back as 1887 Roozeboom* 

 was able to predict that the diagram of condition for sulphur would 

 be as shown on the next page. 



Prof. Tammann has supphed the corroboration of the existence of 

 the triple point. 



Suppose that we have sulphur at a pressure of about 1500 kg./sq. 

 cm. (9*52 tons/sq. inch) and raise its temperature to about 160° 0. 

 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 Hkely has some bear- 

 ing on the occurrence of native crystals of octahedral sulphur. 



It is not every substance which has such sharply defined proper- 

 ties as sulphur, though even these are not as sharp as they might be, 

 owing to the constant presence of amorphous sulphur. An instructive 

 case is afforded by phenol. As the diagram shows, tliere is a consider- 

 able region of the field in which two kinds of crystals of different 

 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 ap- 



teristic. Another slide is prepared, but this time all the sulphur is melted, 

 and can generally be undercooled so far that it crystallises in what is believed 

 to be the octahedral system. This slide is then placed in the projection 

 microscope, when it is seen that its appearance is totally different from that of 

 the first slide. The preparation is then heated on the hot stage, and when 

 the transformation 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 temperature is now raised 

 till about half the preparation has melted, and it is then allowed to cool back 

 a little so as to crystallise. The crystals now show the characteristic mono- 

 clinic crystallisation with brilliant colours, since unmelted monoclinic sulphur 

 is present. 



* Rec. Trav. Ghim. Pays-Bas, vi. 1887, 314. 



