THE HALOGENS 499 



they 'stand in. respect to their atomic and molecular weights. If the 

 weight of the molecule *be large, the substance has a higher specific 

 gravity, a higher melting and boiling point, and a whole series of pro- 

 perties depending on this difference in its fundamental properties. 

 Chlorine in a free state boils at about 35, bromine boils at 60, and 

 iodine only above 180. According to Avogadro-Gerhardt's law, the 

 vapour densities of these elements in a gaseous state are proportional 

 to their atomic weights, and here, at all events approximately, the 

 densities in a liquid (or solid) state are also almost in the rati6 of their 

 atomic weights. Dividing the atomic weight of chlorine (35'5) by its 

 specific gravity in a liquid state (1'3), we obtain a volume = 27, for 

 bromine (80/3-1) 26, and for iodine also (127/4-9) 26. 64 



The metallic bromides and iodides are in the majority of cases, in most 

 respects analogous to the corresponding chlorides* 65 but chlorine displaces 

 ! the bromine and iodine from them, amd bromine liberates iodine frotn 

 iodides, which is taken advantage of in the preparation of these halogens. 

 However, the researches of Potilitzin showed tliat a reverse displace- 

 ment of chlorine by bromine may occur both in solutions and in 

 ignited metallic chlorides in. an atmosphere of bromine vapour that is, 

 a distribution of the metal (according to Berthollet's doctrine) takes 

 place between the halogens, although however the larger portion, still 

 unites with the chlorine, which shows its greater affinity for metals as 

 compared with that of bromine and iodine. 66 The latter, however, 



64 The equality of the atomic volumes of the halogens themselves (a all the more 

 remarkable because in all the halogen compounds the volume augments with the substi- 

 tution of fluorine by chlorine, bromine, and iodine. Thus, for example, the volume of 

 sodium fluoride (obtained by dividing the weight expressed by its formula by its specific 

 .gravity) is about 15, of sodium chloride 27, of sodium bromide 82, and of sodium iodide 41. 

 The volume of silicon chloroform, SiHClj, is 82, and those of the corresponding bromine 

 and iodine compounds are 108 and 122 respectively. The same difference also exists 

 in solutions; for example, NaCl + 200H 2 O has a sp. gr. (at 15/4) of T0106, consequently 

 the volume of the solution '8,658-5/r0106 = 8,620, hence the volume of sodium chloride in 

 solution =8,620 8,603 (this is the volume of 200 ILjO) = 17, and in similar solutions, 

 NaBr = 26andNaI = 85. 



65 But the density (and also molecular volume, Note 64) of a bromine compound is 

 always greater than that of a chlorine compound, whilst that of an iodine compound 

 is still greater. The order is the same in many other respects. For example, an iodine 

 compound has a higher boiling point than- a bromine compound, &c. 



w A. L. Potilitzin showed that in heating vaj:i6us metallic chlorides in a closed tuTje," 

 with an equivalent quantity of bromine, a distribution of the metal between the halogens 

 always occurs, and that the amounts of chlorine replaced by the bromine in the ultimate 

 product are proportional to the atomic weights of the metals taken and inversely proper. 

 Uonal to their equivalence. Thus, if NaCl + Br be taken, then out of 100 parts of 

 chlorine, 5'54 are replaced by the brominej whilst with AgCl + Br 27 '28 parts are 

 replaced. These figures are in the ratio 1 : 4'9., and the atomic weights Na i Ag=l : 4'7. 

 In general terms, if a chloride MCl n be taken, it gives' with nBr a percentage sub- 

 etitution -4M/n 8 , where M is the atomic weight of the metal. This law was deduced 



