136 MR J. Y. BUCHANAN ON THE 



chlorides of K, Rb, and Cs, approximately the same increment of displacement is 

 produced. Commencing again with our solution of KCl, and replacing the potassium by 

 rubidium, causes an increase in the displacement of 0-336 Gx; when rubidium is 

 replaced by caesium, the increase is 0*455 Gx; replacing potassium by caesium produces 

 an increase of 0791 Gx. If we now consider a solution of i/l6 grm.-mol. of KBr, and 

 replace the potassium by rubidium, there is an increase in the displacement of 0'303 Gx ; 

 replacing the rubidium by caesium causes a further increase of 0"4 1 7 Gx ; or if potassium 

 be replaced by caesium, the increase is 0720Gt. Setting out the numbers in the 

 manner shown below gives a clearer view of the various changes that take place 

 when one element in a compound is replaced by another : — 



K. Diff. Rb. Diff. Cs. 



CI . . 1-684 + 0-336 = 2-o'20 + 0-455 = 2-475 

 Br . . 2-153 + 0-303 = 2-456+0-417 = 2-873 



Diff. 0-469 0-436 0-398 



It will be seen that the difference between RbCl and RbBr is nearly the same as 

 that between KCl and KBr, because the increase in the increment of displacement 

 produced when K is replaced by Rb in KCl is about the same as that produced when 

 Rb takes the place of K in KBr. Therefore, when in solutions containing 1/16 grm.- 

 mol. of KCl or KBr the potassium is replaced by rubidium, approximately the same 

 increase in the increment of displacement is produced ; further, when CI is replaced by 

 Br in KCl and RbCl, the increase is nearly the same in each case, but of a higher value 

 than when the change is made in the metals. The difference between the atomic weights 

 of CI and Br is 44*5 ; between K and Rb it is 46 '4. The atomic weight of CI is less 

 than that of K ; so, also, is the atomic weight of Br less than that of Rb ; yet there is a 

 greater difference of displacement produced by changing the acid than by changing 

 the base. 



Turning our attention now to the chlorides and bromides of rubidium and caesium, 

 we find that replacing CI by Br in RbCl causes an increase in the displacement of RbCl 

 which is the mean of the increases produced when Cs replaces Rb in RbCl and RbBr, 

 and is greater than when Br replaces CI in CsCl. There is a greater effect produced by 

 changing the metals when CI is the acid than when Br is the acid ; also, when the acid 

 united with the same base is changed, the variation is greatest for the lightest metal, 

 and least for the heaviest. 



Proceeding on the lines set forth above, we will next consider the changes caused by 

 replacing bromine by iodine, when combined with the same three metals : — 



K. Dilf. Rb. Diff. Cs. 



Br . . 2-153 + 0-303 = 2-456 + 0-417 = 2-873 

 I . . 2-772 + 0-431 = 3-203 + 0-472 = 3-675 



Diff. 0-619 0-747 0-802 



