120 
DR. WALTER WAHL: PHYSICO-CHEMICAL DETERMINATIONS 
possible to carry out by the volumetric methods of Tammann in those cases where 
the volume difference l3etween the diflerent phases is considerable. This is mostly 
the case with regard to the crystallization and melting of the solid form, and the 
volumetric method is therefore nearly always applicable to the determination of the 
melting-point curves. In the case of the transition of crystalline foiius, one into 
another, many instances are known where the transition occurs with only slight 
change of volume, and in such cases a volumetric method, of course, is not applicable. 
Tlie same remarks apply to the thermometric method of the determination of 
transition-points, though this metliod has not until now been at all employed at 
pressures differing from the atmospheric. In cases like tliat aboA’e referred to, the 
simplest method of studying the transition phenomena at ordinary pressure is the 
optical, and tiiis method has further the advantage over the volumetric and the 
thermometric methods that only a quite small quantity of the substance is needed, 
which in many cases, of course, is of essential importance. Further, the change of 
the optical characters of a substance at the transition-point between two crystalline 
forms is generally much more striking than the volume change or the evolution of 
the latent heat, and this is probably the chief cause why optical observation has been 
used in most investigations at ordinary temperatures. The well-known polarization- 
microscope, with a device for heating, constructed by IjEHMANN, has been the 
principal instrument employed. 
The above considerations, togetlier with the desiraldlity of determinations of the 
optical properties of crystals and liquids, especially of those of liquids (refractive 
index) being undertaken at homogeneous pressures other than the atmospheric, led 
the present writer to undertake the working out of methods of optical determinations 
at high pressures and varying temperatures, that is, methods for the optical 
investigation both of tlie boundary lines in the diagram of state of a substance, and 
of the variation of the optical properties of tlie different phases within their existence 
fields. 
Part I.— Apparatus for Optical Determinations at High Pressure. 
The apparatus consists of three chief units :— 
(1) The plant for the production ami measurement of high pressures; 
(2) Tlie “pressure-bomb” to hold the substances under investigation; 
(3) The optical installation for observation and for optical measurements. 
These may conveniently be described apart. 
1. Tltc High-pressure Plant. 
The apparatus for producing the pressures consists of two screw-compressors—one 
for pressures up to G,000 atmospheres, and one for pressures not exceeding 600 
