THE HALOGENS 489 



is simpler to add a small quantity of manganese peroxide arid sulphuric 

 acid to the mother liquid direct. This sets free a portion of the chlorine,. 

 and this chlorine liberates the bromine. Bromine is a dark brov'ii 

 liquid, giving brown fumes, and having a poisonous suffocating smell, 

 from whence its name (from the Greek /fyw/xos, signifying evil smelling). 

 The vapour density of bromine shows that its molecule is Br 2 . In the 

 cold bromine freezes into brown-grey scales like iodine. The melting 

 point of pure bromine is 7*05. r ' 9 The density of liquid bromine at 

 is 3-187, and at 15 about 3-0. The boiling point of bromine is about 

 58'7 ; it is generally purified by distillation. Bromine, like chlorine, i& 

 soluble in water; 1 part of bromine at 5 requires 27 parts of water, 

 and at 15 29 parts of water. The aqueous solution of bromine is of 

 an orange colour, and when cooled to 2 yields crystals containing 10 

 molecules of water to 1 molecule of bromine. 60 Alcohol dissolves a 



39 There has long existed a difference of opinion as to the melting point of pure 

 bromine. By some (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^, in mm. at t was found to be 



p= 20 25 30 85 40 45mm. 



t= -16-6 -14 -12 -10 - 8-5 - 7 



For liquid bromine 



p= 50 100 200 400 600 760mm. 



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 ( + 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 85 40 45 



273 + = 25(>-4 259 261 263 264'6 266 



278 + ' = 295-8 >>W) 302'1 804'8 807'2 809'3 



c = 1-152 1-154 1-157 1-159 1'lGl 1'168 



where t' indicates the temperature of water corresponding with a vapour tension^;, and 

 where c is the ratio of 273 + 1' to 278 + 1. The magnitude of c is evidently expressed with 

 great accuracy by the straight line c = ri708 + 0'0011f. In exactly the same way we find 

 the ratio for liquid bromine and water to be c l = T1585 + 0'00057^. The intersection of 

 these straight lines in fact corresponds with 7'0<P, 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. 



60 The observations made by Paterno and Nasini (by Raoult'a method, Chapter I. 

 Note 49) on the temperature of the formation of ice (-1-115, with 1-891 grams of bromine 



