132 
DR. WALTER WAHL: PHYSICO-CHEMICAL DETERMINATIONS 
melting substances.* A practical consequence of this difference between the ordinaiy 
“isobaric” melting at constant pressure and the “isothermic” melting, is that the 
melting-point can he instantly passed in either direction, and regions of a different 
degree of super-cooling and a correspondingly different degree of velocity of 
crystallization and of rate of spontaneous formation of crystal-nuclei can be experi¬ 
mentally reached and studied. In nearly all the cases investigated observations 
bearing upon the above have been made, and the more important will be quoted in 
connection with the description of the determination of the diagrams of state. 
5. The Diagram of State of Carbon Tetrabromide CBiq. 
Lehmann first ol^served that carbon tetrabromide, which crystallizes from the molten 
state in growth-structures belonging to the cubic system and appearing quite dark 
between crossed nicols, on further cooling down, changes into highly double-refracting 
crystals.! Schwarz found that this transition from one crystalline form to another 
takes place at 46°’I C.| 
This transition-point, and the influence of admixtures of carbon tetrachloride on it, 
has since been very carefully studied by Rothmund Ijy means of a thermometric 
method. § Rothmund gives the transition-point of pure carbon tetrabromide as 
46°’9I C. The melting-point is 92° C. The double-refracting modification of carbon 
tetrabromide crystallizes also from solutions in acetone. The crystals are monoclinic, 
but have crystal angles which differ very little from those of the cubic octahedron ; 
they have been measured by Zirngiebl.|| 
In order to determine the diagram of state of carbon tetrabromide a small quantity 
of it was brought into the pressure-bomb between two round object-glasses in the 
manner described on p. 130. The carbon tetrabromide had been purified by recrystal¬ 
lization from petrol-ether, and finally by sublimation in a charcoal vacuum from room 
temperature to that of liquid air. 
(l) Melting-point Determinations. —When the pressure is increased at temperatures 
above 92° C.—the melting-point at ordinary pressure—crystalline growth-structures 
suddenly appear and grow rapidly. At constant temperature this does not always 
take place at exactly the same pressure, and these pressures at which the rapid 
growth, in the form of growth-structures, begins are not identical with the pressure 
corresponding to the melting-point, but slightly higher. At the melting-point 
pressure the crystals grow only slowly, and the rate of growth is about equal in all 
directions, so that compact crystals result, and no growth-structures; but if the 
* A. Ij. Day and E. T. Allen, ‘ Publications of the Carnegie Institution,’ Washington, No. 31, p. 57 
(1905); G. Tammann, ‘ Zeit. f. Phys. Chem.,’ 68, p. 257 (1910). 
t 0. Lehmann, ‘ Molekularphysik.,’ L, p. 178 (1888). 
I W. Schwarz, ‘ Preisschrift Gottingen,’ 1892, p. 47. 
§ V. Rothmund, ‘Zeitschr. f. Phys. Chem.,’ 24, p. 705 (1897). 
II Quoted l)y Groth, ‘ Chem. Kristallographie,’ L, p. 230. 
