THE HALOGENS 493 



Tespects analogous to the corresponding chlorides, 05 but chlorine displaces 

 the bromine and iodine from them, and bromine liberates iodine from 

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

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

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



fis But the density (and even 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 com- 

 pound has a higher boiling point than a bromine compound, Arc. 



66 A. L. Potilitzin showed that in heating various metallic chlorides in a closed tube, 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 proportional to 

 their equivalency. Thus, if NaCl + Br be taken, then out of 100 parts of chlorine, 5'54 are 

 replaced by the bromine, whilst if AgCl + Br be taken, the 27'28 parts are replaced. 

 These figures are in the ratio 1 : 4'9, but the atomic weights Na : Ag = l : 4'7. In general 

 terms, if a chloride MCl n be taken, then it gives with ?iBr a percentage substitution 

 = 4M/M 2 , where M is the atomic weight of the metal. This law was deduced from observa- 

 tions on the chlorides of Li, K, Na, Ag (n= I), Ca, Sr, Ba, Co, Ni, Hg, Pb (n = 2), Bi (n = 8), 



In these determinations of Potilitzin we see not only a brilliant confirmation of 

 Berthollet's doctrine, but also the first effort (there have been no similar determinations 

 since 1879) to directly determine the affinities of elements by means of displacement. The 

 chief object of these researches consisted in proving whether a displacement occurs in 

 those cases where heat is absorbed, and in this instance it should be absorbed, because 

 the formation of all metallic bromides is attended with the evolution of less heat than 

 that of the chlorides, as is seen by the figures given in Note 55. Among the results of 

 other researches made by Potilitzin in the same direction, it is necessary to turn atten- 

 tion to the following. 



If the mass of the bromine be increased, then the amount of chlorine displaced also 

 increases. For example, if masses of bromine of 1 and 4 equivalents act on a molecule of 

 sodium chloride, then the percentages of the chlorine displaced will be 6'08 p.c. and 

 12'46 p.c. ; in the action of 1, 4, 9, 16, 25, and 100 molecules of bromine on a molecule of 

 barium chloride, there will be displaced 7'8, 17'6, 23'5, Sl'O, 35'0, and 45'Op.c. of chlorine. 

 If an equivalent quantity of hydrochloric acid act on metallic bromides in closed tubes, and 

 in the absence of water at a temperature of 800, then the percentages of the substitution 

 of the bromine by the chlorine in the double decomposition taking place between univalent 

 metals are inversely proportional to their atomic weights. For example, NaBr + HC1 gives 

 at the limit 21 p.c. of displacement, KC1 12 p.c. and AgCl 14^ p.c. Essentially the same 

 takes place in an aqueous solution, although the phenomena is complicated by the parti- 

 cipation of the water. The reactions proceed of themselves in one or the other direction 

 at the ordinary temperature but at different rates. In the action of a dilute solution (1 

 equivalent per 5 litres) of sodium chloride on silver bromide at the ordinary tempera- 

 ture, 2*07 p.c. of bromine is replaced in six and a half days, with potassium chloride 

 1*5 p.c. With an excess of the chloride the magnitude of the substitution increases ; 

 with four equivalents it is 4'95 p.c. in nine days. In the action of 9 grams of sodium 

 chloride on I'l grams of silver bromide, 9'G9 p.c. of bromine is replaced in thirteen days. 



