AT HIGH PRESSURES BY OPTICAL METHODS. 
119 
between solids and liquids and solids and solids is due to the work of Tammann 
carried out at Dorpat and later at Gottingen.* 
Tammann has used two different volumetric methods; working partly at constaiit 
temperature and altering the pressure, and partly at constant pressure altering the 
temperature, hut the former method was chiefly employed. Only in the case of 
Bismuth, Tin, and Lead, Tammann has used a tliermometric method. Tlie actual 
detennination of pressure extends in Tammann’s work to between 3,000 and 
4,000 kg./cm.^, hut in a few instances melting - point temperatures have l>een 
reached which correspond to extrapolated pressui’es of between 5,000 and 10,000 
atmospheres. 
Only quite recently Bridgman has published a series of measurements on the 
melting-point of mercury at pressures from 1 to 12,000 kg./cm.hf The change of the 
state of the mercury was observed by three different metliods, one electric and two 
volumetric. The pressures were measured by the change of the electrical resistance 
of a mercury thread and of manganln wire ; the influence of pressure upon the 
resistance having first been determined by standardizing with absolute manometers 
of the Amagat type, constructed by Bridgman, and rendering possible the exact 
measurement of pressures up to G,800 and 12,000 kg./cm.h;|; At pressures above 
12,000 kg./cm.^ the determination of the pressure in Bridgman’s work is entirely 
based upon the change of the resistance of manganin wire, Jiut Bridgman 
has been able to measure accurately pressures as liigh as 20,000 kg./cm.^ by this 
method, and states that he occasionally has reached pressures as high as 
40,000 kg./cm.^. 
Of a recent date are also the determination of the melting-points of Pb, Sn, Bi, and 
Cd by the thermometric method at pressures from 1 to 2,000 atmospheres, undertaken 
at the geophysical laboratory of tlie Carnegie Institution of Wasliington, D.C.,§ 
and Cohen’s investigation. of the transition of ZnSO^-hflGO at pressures up to 
1,500 kg./cm.^ by means of electrical measurements. 1| 
One of tbe most interesting results of Tammann’s high-jiressure investigations is 
the discovery of the occurrence of crystallized modifications which are stable only at 
high pressui‘e, as, for instance, in the case of phenol, water, methylene iodide, and 
silver iodide. Of great interest also is the tracing of the boundary lines between 
the different crystalline modifications and between these and the liquid phase in the 
diagram of state, viz., the fixing of the triple points. These inquiries it has been 
* G. Tammann, ‘Ann. d. Physik,’ 68, pp. 553, 629 (1899). ‘ Kristallisiercn u. Schmelzen,’ Hamburg, 
1903. ‘Zeitschr. f. Phys. Chem.,’ 69, p. 569 (1909); 72, p. 609 (1910); 75, pp. 75-733 (1910). 
‘ Zeitschr. f. Anorg. Chem.,’ 40, p. 54 (1904); 63, p. 285 (1909). 
t P. W. Bridgman, ‘Proc. Amer. Acad. Arts and Sciences,’ 47 (12), p. 377, December (1911). 
f P. W. Bridgman, ‘Proc. Amer. Acad. Arts and Sciences,’ 44 (8), p. 201; 44 (9), p. 221; 47 (11), 
p. 321. 
§ J. Johnston and L. H. Adams, ‘Amer. Journ. of Science,’ 4 ser., XXXI., p. 501. 
II E. Cohen, ‘Zeit. f. Phys. Chem.,’ 75, p. 1 (1911). 
