November iS, 



NA TURE 



83 



By far the grcaler part uf our infonnalion as to the 

 quantitative relations of bodies at high pressures we owe 

 to Prof. Gustave Tammann, who has collected his results 

 in a book entitled " Kristallisiercn und Schmelzen," the 

 advent of which (1003) must be regarded as an important 

 event in the history of the subject. 



SOOO 



Fig. I.— Full lines indi 



The complete thermodynamic specification of a body 

 involves a knowledge of its mass, volume, pressure, tempera- 

 ture, energy, entropy', surface tension, and nature, whether 

 liquid, solid, glassy, crystalline, or amorphous. 



Prof. Tammann has simultaneously measured the 

 pressure, temperature, volume, and mass of many substances 

 under high pressure, and at temperatures extending from 

 80° C to 200° C. — taking cognisance of the physical state — ■ 

 and has thus been able to plot out many interesting 

 diagrams of condition. The apparatus consists of a screw 

 press by which a piston of ebonite is driven down a steel 

 cyhnder of small known cross-section. The cylinder is 

 filled with oil, and the ebonite piston fits practically oil- 

 tight. The oil communicates with the oil contained in a 

 strong steel vessel, wliich also encloses a glass tube open 

 at the lower end, containing the substance and dipping 

 below the level of mercury contained in a dish. The oil 

 occupies the rest of the space. The steel vessel is placed 

 in a thermostat so that its temperature can. be ascertained. 

 ■| H" nil pressure is measured by a Bourdon gauge, which it 

 possible to standardise, thanks to the previous work 

 \inagat and Tait. In order to construct a diagram of 

 ^..iiidition, it is necessary and sufficient to find a number 

 of points separating the liquid from the solid area, or 

 separating the areas corresponding to different crystalline 

 forms in the case where the transformation of one sort 

 of crystal into another is under investigation. To under- 

 stand how this is done, it is best to take a special case. 

 If we have a quantity of a substance under a known 

 pressure and temperature in the piezometer, and suddenly 

 increase the pressure, so that there is not time for heat 

 to pass in or out to any appreciable extent before the 

 pressure gauge can be read, we have practically adiabatic 

 compression. If the apparatus be then left to itself, the 

 heat which we may suppose to be liberated by the pressure 

 will slowly diffuse outwards, and the pressure will fall as 

 time goes on. If we happen to start from a point on the 

 ni.p. curve before the pressure is raised, then the final 

 result will be that we shall thaw or freeze more or less 

 of the material, and the original pressure will be exactly 

 regained, the change of state compensating the impressed 

 change of volume. If, however, the increase of pressure 

 has been so great that a change of state of the whole mass 

 has been brought about, then the after variation of pressure 

 will be so much greater that it is easy to distinguish this 

 case from the previous one. 



The accompanying diagram (Fig. 2), taken from Prof. 

 N'O. 2090, VOL. 82] 



Tammann 's book, shows how the equilibrium curve can 

 be located in the case of carbon dioxide and naphthalene. 

 In the former case the temperature was 0-31° C. The 

 pressure w^as 3800 kilograms per sq. cm., or 24-13 tons 

 per sq. inch. (157-49 kilograms per sq. cm. = i ton per 

 sq. inch=i52-3S atmospheres.) 



The pressure was raised adiabatic- 

 ally to 4400 kg./cm.^ (27-93 tons/sq. 

 inch), and the subsequent fall of 

 pressure plotted against a time scale 

 for ten minutes. The pressure was 

 then adiabatically reduced to 3550 

 kg./cm.^, and the recovery curve 

 again plotted. The equilibrium pres- 

 sure must lie between the pressures 

 approached asymptotically on the 

 diagram, i.e. between 3825 and 3792 

 kg. /cm.-. A repetition between 

 narrower pressure Mmits enables the 

 pressure to. be fixed at between 380S 

 and 3797 kg. /cm.". .A similar pro- 

 cedure fixed the pressure of the m.p. 

 of naphthalene between 3090 and 30S0 

 kg. /cm. at the temperature con- 

 sidered, a difference which corresponds 

 to 02° C, the actual temperature 

 possibly differing from the thermostat 

 temperature by 0-1° C. 



We may now pass on to the con- 

 sideration of some of the results 

 obtained, which refer, not only to 

 change of melting points, but to 

 changes in the temperatures of trans- 

 formation of isomorphic forms. 

 As illustrations of such changes, I 

 show here the transformation of yellow to red mercuric 

 iodide, which shows well in the projection microscope ; 

 also .Mitscherlich's transformation of potassium bichromate, 

 and sulphur in two forms.' 



4000 



3000 



2000 



1000 



4400 



E 



3800 





2600 



20 30 



Minute s 



1 Experimental Demonstration of a Transformation of Sulphur.— 

 A microscope slide is prepared by partially melting a fragment of monocHnic 

 sulphur, and enclosing some of the melt between the slide and cover-slip, 

 well pressed together. The presence of unmelted monocHnic sulphur insures 

 the crystallisation of this variety on loweiing ihe temperature. By means 

 of a hot stage it is possible to preserve ihe crystallisation long enough to 

 ,s of a projection polarising microscope. The appearance 

 tic. Another slide is prepared, but this lime all the sulphur 

 generally be undercooled so far that it crystalli; 



xhibit it by i 

 i very characte 

 i melted, and c; 



I what 



