THE HALOGENS 487 



mid iodine, is much more perfect. Not only have their hydrates or 

 halogen acids much in common, but they themselves resemble chlorine 

 in many respects,"' 5 and even the properties of the corresponding 

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

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

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

 aluminium, magnesium, and barium are soluble in water, like 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. Besides which even the oxygen compounds of bromine 

 and iodine 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 



i5 For instance, the experiment with Dutch metal foil (Note 16) may be made with 

 bromine just as well as with chlorine. A very instructive experiment on the direct com- 

 bination of the halogens with metals may be made by throwing a small piece (a shaving) of 

 aluminium into a vessel containing liquid bromine ; the aluminium, being lighter, floats 

 on the bromine, and after a certain time reaction sets in accompanied by the evolution 

 of heat, light, and fumes of bromine. The incandescent piece of metal moves rapidly 

 over the surface of the bromine in which the resultant aluminium bromide is dissolved. 

 It was in this manner that Gustavson prepared that mixture of bromine and aluminium 

 bromide which reacts by metalepsis with the greatest ease in those cases when bromine 

 by itself is not able to bring about metalepsis, or else only acts very slowly, as, for 

 instance, with benzene, C 6 H 6 . When drops of this hydrocarbon are added to bromine 

 containing aluminium bromide, they immediately give a mass of hydrobromic acid and 

 of the product of metalepsis. Gustavson showed that the cause of this facility of 

 reaction must be looked for in the capacity of aluminium bromide to form an unstable 

 compound with the products of reaction which are formed. For the sake of comparison 

 we will proceed to cite several thermochemical data (Thomsen) for analogous actions of 

 (1) chlorine, (2) bromine, and (3) iodine, with respect to metals; the halogen being 

 expressed by the symbol X, and the sign plus 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 : 



123 



K.j + X 3 211 191 160 



195 172 138 



59 45 28 



83 68 48 



Hg +X 2 63 51 34 



Ca + X 3 170 141 



Ba + X 3 195 170 



Zn + X 3 1)7 76 49 



Pb +X 3 83 64 40 



Al +X 3 161 120 70 



We may remark that the latent heat of vaporisation of the molecular weight Br 3 is about 

 7'ii, and of iodine 6'0 thousand heat units, whilst the latent heat of fusion of Br 3 is about 

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

 between 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 ZnI 2 would give 

 -48 + 8 + 3, or about sixty thousand heat units, or li times less than Zn + Cl a , 



