436 PRINCIPLES OF CHEMISTRY 



The mixture was always formed with equivalent quantities for in- 

 stance, 4BC1 3 + 3CBr 4 . It appeared that there was no exchange what- 

 ever on simple intermixture, but that it proceeds, and then slowly, 

 when the mixture is heated (for example with the above-cited mixture 

 at 123 4-06 per cent, of Cl was replaced by Br after 14 days' heating, 

 and 6'83 per cent, after 28 days, and 10*12 per cent, when heated at 

 150 for 60 days) A limit was always reached which corresponded 

 with the complemental system ; for example, in the given instance to the 

 system 4BBr 3 + 3CCl 4 . So in this last 89*97 per cent, of bromine in 

 the BBr 3 was replaced by chlorine ; that is, there was obtained 89 '97 

 molecules of BC1 3 and there remained 10*02 molecules of BBr 3 , and 

 therefore the same state of equilibrium was reached as that given by 

 the system 4BC1 3 + 3CBr 4 . Both systems gave one and the same state 

 of equilibrium at the limit, as follows from Berthollet's doctrine. 28b 



- 8b G. G. Gustavson's researches, which were conducted in the laboratory of the 

 St. Petersburg University in 1871-72, are among the first in which the measure of 

 the affinity of the elements for haloids appears with perfect clearness in the limit of 

 substitution and in the rate of reaction. The researches conducted by A. L. Potilitzin 

 (of which mention will be made in Chapter XI. Note 6(i) in the same laboratory touch on 

 another aspect of the same problem which has not yet made much progress, notwith- 

 standing its importance and the fact that the theoretical side of the subject (thanks 

 especially to Guldberg and Van't Hoff ) has since been rapidly pushed forward. It would 

 be very important if the researches of Gustavson touched on the influence of mass, and 

 were more fully supplied with data concerning velocities and temperatures, because of 

 the great significance which the instance considered has for the understanding of double 

 saline decompositions in the ' absence of water.' 



Furthermore, Gustavson showed that the greater the atomic weight of the element 

 (B, Si, Ti, As, Sn) combined with chlorine the greater the amount of chlorine replaced 

 by bromine by the action of CBr.j, and consequently the less the amount of bromine re- 

 placed by chlorine by the action of CC1 4 on bromine compounds. For instance, for 

 chlorine compounds the percentage of substitution (at the limit) is 



BC1- SiCl, TiCl 4 AsCl 3 SnCl 4 



10-1 12-5 43'6 71-8 77'5 



It should be observed, however, that Thorpe, on the basis of his experiments, denies 

 the universality of this. I may mention one conclusion which it appears to me may be 

 drawn from the above cited figures of Gustavson, if they further verify themselves even 

 within narrow limits. If CBr 4 be heated withKCl,, then an exchange of the bromine by 

 chlorine takes place. But what would be the result if it were mixed with CC1 4 ? Judging 

 by the magnitude of the atomic weights, B = 11, C = 12, Si = 28, about 11 p.c. of the chlorine 

 would be replaced by bromine. But what does this signify ? I think that this shows the 

 existence of a movement of the atoms in the molecule. The mixture of CC1 4 and CBr 4 

 does not remain in a state of static equilibrium ; not only are the molecules contained in 

 it in a state of movement but also the atoms in the molecules, and the above figures show 

 the measure of their translation under these conditions. The bromine in the CBr 4 is, 

 within the limit, substituted by the chlorine of the CC1 4 in a quantity of about 11 out of 

 100 ; that is, a portion of the atoms of bromine previously to this moment in combination 

 with one atom of carbon pass over to the other atom of carbon, and the chlorine passes over 

 from this second atom of carbon to replace it. Therefore, also, in the homogeneous mass 

 CC1 4 all the atoms of Cl do not remain constantly combined with the same atoms of 



