234 REPORT— 1897. 



dissolved matter. It must be an independent motion, and the ions must 

 be dissociated from each other. It will be noticed that there is nothing 

 to show that the ions are not combined with solvent molecules, and there 

 seems reason to suppose that such may be the case. 



We may conclude, from the experimental confirmation described 

 above, that the velocity of an ion, as calculated by Kohlrausch's theory 

 from the conductivity, really does represent the actual speed with which, 

 on the average, the ion makes its way through the solution. We can 

 therefore apply the theory with confidence to cases in which the experi- 

 mental confirmation would be difficult or impossible. 



If we know the specific velocity of any one ion, we can, from the con- 

 ductivity of very dilute solutions, at once deduce the velocity of any other 

 ion with which it may be combined, without having to determine the 

 migration constant of the compound, which is a matter involving consider- 

 able trouble. Thus, taking the specific ionic velocity of hydrogen as 

 0'0032 cm. per second, we can, by determining the conductivity of dilute 

 solutions of any acid, at once find the specific velocity of the acid radicle 

 involved. Or, again, since we know the specific velocity of the silver ion, 

 we can find the velocities of a series of acid radicles at great dilution by 

 measuring the conductivity of their silver salts. 



By such methods Ostwald, Bredig, and other observers have found the 

 specific velocities of many ions both of inorganic and organic compounds, 

 and examined the relation between constitution and ionic velocity. A 

 full account of such data will be found in a paper by Bredig in vol. xiii. of 

 the ' Zeitschrift fiir physikalische Chemie,' p. 191. The velocities are 

 calculated from the conductivities measured in terms of mercury units, 

 and so must be multiplied by 110 x 10~' if they are wanted in centimetres 

 per second. 



The velocity of elementary ions is found to be a periodic function of the 

 atomic weight, similar elements lying on similar portions of the curve. 

 The curve much resembles that giving the relation between atomic weight 

 and viscosity in solution. For complex ions the velocity is largely an 

 additive property ; to a continuous additive change in the composition of 

 the ion corresponds a continuous but decreasing change in the velocity. 

 Thus Ostwald's results for the anions of the formic acid series give 



HjCjO, 38-3 



H,C,0, 34-3 



3V.2W„ u^-3j _ 4.Q 



Formic acid . . HCO™ 51-2} _i2-9 



Acetic „ TT ^, ,^ nn „ 



Propionic „ 



Butyric ,, 



Valeric „ . . H'C.^0.," 28-8; - 2-0 



Caproic „ . . H„Csd2 27-4} - 1-4 



Diff. for CHj 



ILc'a 30-8[ "" 3-5 



Bredig finds similar relations for every such series of compounds which 

 he examined. Isomeric ions of analogous constitution have equal 

 velocities. A retarding effect is, in general, produced by the replacement 

 of H by CI, Br, I, Me, NH2 or NO2 : of any element by an analogous one 

 of higher atomic weight (except O and S) ; of NH3 by H2O ; of (CN)6 by 

 (0204)3 ; by the change of amines into acids ; of sulphonic acids into 

 carboxylic acids ; acids into cyanamides, dicarboxylic into monocarboxylic 

 acids ; and by monamines into diamines. The additive effect is, however, 

 largely influenced by constitution. Thus in metamerides the velocity 

 increases with the symmetry of the ion, especially as the number of 

 C— N unions gets greater. 



