210 REPORT—1890. 
has remarked that the motion under an E.M.F. is constrained, whereas the 
motion without E.M.F. is free ; and the difference of the two cases with 
respect to the energy required is thus explained. 
Many of the observed phenomena of electrolysis are most easily 
explained on the assumption of a permanent dissociation of at least a 
portion of the electrolyte into component parts which become ‘ions’ (i.e. 
move with the positive and negative electricity respectively) when an 
electromotive force acts upon the electrolyte. If we may picture to our- 
selves the whole number of molecules taking part in dissociation and 
frictionless recombination, being combined molecuies for a certain fraction 
of every instant and dissociated ‘ions’! for the remainder, the average re- 
sult for the whole electrolyte will be the same as if the same fraction of 
the whole number of molecules were permanently combined, the remainder 
being permanently dissociated. There does not seem to be any experi- 
mental method of distinguishing between these two alternatives, and in 
default of experimental evidence for the one or other we may provision- 
ally adopt whichever we please. But it.may be well to accentuate here 
what Arrhenius (‘Zeitschr. f. phys. Chemie,’ i. p. 638) has already men- 
tioned, namely, that the term ‘ dissociation,’ as here used, is liable to be 
misunderstood and confounded with the same term as applied, for instance, 
to the resolution of an armmonium salt into two separate bodies at a high 
temperature. As referring to electrolysis, dissociation means the separa- 
tion of a molecule into atoms or their equivalents, and would only corre- 
spond to ordinary dissociation if atoms of the same kind were collected 
and set free from the liquid. Thus one need not expect a solution of KCl, 
even though all the salt were dissociated into K and Cl atoms, to smell 
of chlorine until one has done the work necessary to accumulate the 
electrified chlorine atoms and produce molecular chlorine; in other 
words, until the solution has been electrolysed. Free chlorine and dis- 
sociated chlorine ions are not by any means to be regarded as identical 
in physical state. In the electrolytic sense the conception of dissociation 
is new to science, and the numerical results obtained from its use are the 
more startling, as those compounds which we have been accustomed to 
regard as most capable of resisting dissociation in the ordinary sense are 
precisely those which are electrolytically most completely dissociated.” 
Quite recently the dissociation theory has been put in such a form as 
renders it possible to express numerically the fraction of the whole 
number of molecules which are dissociated in the formation of an electro- 
lyte by solution of a salt in water. The first development of the theory 
is mainly due to Arrhenius. In Part II. of a memoir? presented to the 
Academy of Sciences of Sweden, June 6, 1883, on the ‘ Chemical Theory 
of Electrolytes,’ he explains the action of a very large number of chemical 
changes in solutions on the assumption of a coefficient of activity for 
each acid or base, representing the ratio of the number of active or dis- 
sociated molecules to the whole number of molecules of salt in the 
solution, the action of the solvent being assumed to be merely to dissociate 
the salt to a greater or less extent. This ratio is taken to be identical 
* Mr. J. Brown takes exception to the use of the word in this sense. It avoids 
circumlocution, however, and stands for ‘ those parts of a molecule which would 
become ions if an E.M.F. acted.’ A new name might be found for them if necessary. 
* See Armstrong, Electrician, Aug. 26, 1887, and on the other side Ostwald, 
Zeitschr. fiir phys. Chem. ii. p. 270 (1888). 
* See B.A. Report, 1886, p. 357. 
