Note 59 — Chapter XI · THE HALOGENS: CHLORINE, BROMINE, IODINE, AND FLUORINE

There has long existed a difference of opinion as to the melting point of pure bromine. By some investigators (Regnault, Pierre) it was given as between -7° and -8°, and by others (Balard, Liebig, Quincke, Baumhauer) as between -20° and -25°. There is now no doubt, thanks more especially to the researches of Ramsay and Young (1885), that pure bromine melts at about -7°. This figure is not only established by direct experiment (Van der Plaats confirmed it), but also by means of the determination of the vapour tensions. For solid bromine the vapour tension p in mm. at t was found to be—

p = 20 25 30 35 40 45 mm.

t = -16°·6 -14° -12° -10° -8·5° -7°

For liquid bromine—

p = 50 100 200 400 600 760 mm.

t = -5°·0 +8°·2 23°·4 40°4 51°·9 58°·7

These curves intersect at -7°·05. Besides which, in comparing the vapour tension of many liquids (for example, those given in Chapter II., Note 27), Ramsay and Young observed that the ratio of the absolute temperatures (t + 273) corresponding with equal tension varies for every pair of substances in rectilinear proportion in dependence upon t, and, therefore, for the above pressure p, Ramsay and Young determined the ratio of t + 273 for water and bromine, and found that the straight lines expressing these ratios for liquid and solid bromine intersect also at 7°·05; thus, for example, for solid bromine—

p = 20 25 30 35 40 45

273 + t = 256·4 259 261 263 264·6 266

273 + t′ = 295·3 299 302·1 304·8 307·2 309·3

c = 1·152 1·154 1·157 1·159 1·161 1·163

where t′ indicates the temperature of water corresponding with a vapour tension p, and where c is the ratio of 273 + t′ to 273 + t. The magnitude of c is evidently expressed with great accuracy by the straight line c = 1·1703 + 0·0011t. In exactly the same way we find the ratio for liquid bromine and water to be c₁ = 1·1585 + 0·00057t. The intersection of these straight lines in fact corresponds with -7°·06, which again confirms the melting point given above for bromine. In this manner it is possible with the existing store of data to accurately establish and verify the melting point of substances. Ramsay and Young established the thermal constants of iodine by exactly the same method.