494 PRINCIPLES OF CHEMISTRY 



metallic compounds of bromine and iodine are very much alike. Thus, 

 the chlorides, bromides, and iodides of sodium and potassiujn crystallise 

 in the cubic system, and are soluble in water j the chlorides of calcium, 

 aluminium, magnesium, and barium are just as soluble in water as the 

 bromides and iodides of these metals. The iodides and bromides of 

 silver and lead are sparingly soluble in water, like the chlorides of 

 these metals. The oxygen compounds of bromine and iodine also 

 present . a very strong analogy to the corresponding compounds of 

 chlorine.- A hypobromous acid is known corresponding with hypo- 

 chlorous acid. The salts of this acid have the same bleaching property 

 as the salts of hypochlorous acid. Iodine was discovered in 1811 by 

 Courtois in kelp, and was shortly afterwards investigated by Clement, 

 Gay-Lussac, and Davy. Bromine was discovered in 1826 by Balard 

 in the mother liquor of sea water. 



Bromine and iodine, like chlorine, occur in sea water in combina- 

 tion with .metals. However, the amount of bromides, and especially 

 of iodides, in sea water is so small that their presence can- only be 

 discovered by means of sensitive reactions. 56 In the extraction of salt 

 from sea water the bromides remain in the mother liquor. Iodine and 

 bromine also occur combined with silver, in admixture with silver 

 chloride, as a rare ore which is mainly found in America. Certain 



over the surface of the bromine in which the resultant aluminium bromide dissolves. 

 For the sake of comparison we will proceed to cite several thermochemical data (Thorn- 

 een) for analogous actions of (1) chlorine, (2) bromine, and (8) iodine, with respect to 

 metals ; the halogen being expressed by the symbol X, and the plus sign connecting the 

 reacting substances. All the figures are given in thousands of calories, and refer to 

 molecular quantities in grams and to the ordinary temperature : 



1 2 8 



a + X 2 211 191 160 



195 172 168 



59 45 28 



83 68 48 



Hg +X 2 63 61 84 



Ca + X a 170 141 



Ba +Xa 195 170 



Zn +X 8 97 76. 49 



Pb +X 9 83 64 40 



Al -t-X, 161 120 70 



"We may remark that the latent heat of vaporisation of the molecular weight Br s is about 

 7'2 and of iodine 6'0 thousand heat units, whilst the latent heat of fusion of Br a is about 

 0'8, and of I 3 about 8'0 thousand heat units. From this it is evident that the difference 

 tetween the amounts of heat evolved does not depend on the difference in physical state. 

 For instance, the vapour of iodine in combining with Zn to form Znlj, would give 

 48 + 8+8, or about sixty thousand heat units, or 1 times less than Zn + C1 3 . 



One litre of sea-water contains about 20 grams of chlorine, and about 0'07 gram 

 o! bromine. The Dead Sea contains about ten times as much bromine. 



