DECEMBER 17, 1896] 
NATURE 
151 
and now Prof. Poynting (P27. A/ag., October) shows in detail 
how the analogies may be accounted for without postulating any 
dynamical similarity. 
There remains the so-called dissociation needed to explain 
electrolysis. It has been long known, however, that a kind of 
instability, or ease of interchange, is all that is necessary, not 
actual permanent dissociation into donstituent atoms. What is 
‘certain is (1) that the atoms of an electrolyte migrate in opposite 
directions, and (2) that they require no appreciable electric force 
to tear them asunder. These are facts, and the instability of 
composition thus evidenced is such as almost to compel the pro- 
visional use of the term. ‘‘ virtual dissociation” ; although that 
condition may very likely be brought about by the loose affinities 
of outlying members of complex molecular aggregates—a con- 
<eptign which Prof. Armstrong himself promulgated as an 
hypothesis, but has not yet, I believe, made definite. 
The problem presented by the fact of molecular combination 
is, I suppose, universally recognised by physicists as a difficult 
but important one, and attempts have been made—by Helmholtz 
among others—to attack it. 
Occasionally it happens that the perennial attempt to reduce 
one province of chemistry after another, to simple dynamics 
ultimately, to thermodynamics or electro-dynamics provision- 
ally, meets with some partial success; but it seldom meets with ap- 
preciation, perhaps not always with apprehension, from professed 
chemists. Thus, for instance, Prof. Armstrong seems to speak 
with some bitterness of ‘‘ those who are now arrogating to them- 
selves the position of superior persons to whom has been granted 
the mission and plenary powers to reform an ancient society long 
steeped in superstition—to wit, the chemists.” 
Does he intend by such phrases to surround the domain of 
chemistry with an adiabatic boundary or barbed wire fence for 
the exclusion of trespassers? I have reason to believe strongly 
that it is not so, and that it is only his wide knowledge and 
reading which tends to make him hypercritical. 
OLIVER J. LODGE. 
As Prof. Armstrong has, in the issue of NATURE dated 
November 26, continued the discussion on the theory of solution, 
may I beallowed to point out that there are two entirely distinct 
questions involved ? 
The first is to investigate the state of a dissolved substance 
and the cause of osmotic pressure, and the second to ask what 
is the physical reason why some solutions possess that property, 
absent from others. by virtue of which they become electrolytes, 
have abnormally great osmotic pressure, and hjgh chemical 
activity. 
The answer to the first and more fundamental question 
remains uncertain. \ Van ’t Hoff’s work showed that there was 
a very close analogy between solution and evaporation, and this 
led to the idea that there might also be a dynamical similarity 
between the condition of a gas and that of a substance dissolved 
in a liquid. Such an hypothesis, although it at once explains 
the application of the gaseous laws to dilute solutions, neglects 
the undoubted similarity between the process of solution and 
certain kinds of chemical action. Prof. Poynting has done a 
good work in showing that, on certain fairly probable assump- 
tions, the phenomena of dilute solutions can be fully explained 
by the opposite idea of aggregation. We have, therefore, two 
hypotheses, each based on an analogy, and each capable of 
accounting for the facts to be explained. It is probable that 
neither of them represents the exact mechanical truth, and I 
imagine that their authors would make no such claim on their 
behalf. While using both views as guides to future work, we 
need commit ourselves to neither. Although, personally, I am, 
at present, inclined to think that some form of chemical theory 
will ultimately be found to be capable of representing the facts, 
I have no wish to express my faith in any definite hypothesis. 
As Prof. Armstrong says, ‘‘ there is no need to be in so great a 
hurry—it is no disgrace to admit that we cannot yet explain all 
the mysteries of the universe.” 
It will be seen that this first question does not at all involve 
the second, which refers to the nature of the fundamental 
property distinguishing an electrolyte from a non-electrolyte. 
Yet Prof. Armstrong uses Prof. Fitzgerald’s warning against 
prejudging the first question, and Lord Rayleigh’s repetition of 
that warning, as reasons for refusing to accept the evidence for 
a particular answer to the second. 
I may repeat what I said in the letter which appeared in your 
NO. 1416, VOL. 55 | 
issue of October 15, that, although there is strong evidence to 
show that the opposite ions are free from cach other, there is 
nothing in the facts of electrolysis inconsistent with the view 
that they are united with solvent molecules, and that solution 
is essentially a chemical process, and the cause of osmotic 
pressure a combination between the solvent and the dissolved 
matter. 
Lord Rayleigh’s words, as quoted by Prof. Armstrong, are : 
‘““Tt isto be hoped that chemists will take into grave considera- 
tion the emphatic warning that Prof. Fitzgerald has given, par- 
ticularly as to the danger of supposing that there is any dynamical 
similarity between the condition of a gas and that of a dissolved 
substance in a liquid. . . . There is possibly a risk of pushing 
analogies too far, and of supposing that there is a real dynamical 
similarity, whereas, perhaps, there is only a similarity in mathe- 
matical law.” 
This statement is clearly meant to apply to the question of 
the fundamental nature of solution—the question whether or 
not the “dissolved matter is to be considered to exist in a con- 
ditior*dynamically similar to the gaseous state. It does not 
refer to the second problem, which is concerned with the nature 
of the property characteristic of an electrolyte. Yet Prof. Arm- 
strong adds: ‘‘I, for one, require no better support than this, 
and shall continue to be, as I have been from the outset, a 
determined opponent of what, I think, may fairly be termed 
the nonsensical hypothesis of ionic dissociation.” 
If the evidence in support of the idea of dissociation is 
not strong enough to carry conviction to Prof. Armstrong’s 
mind, the supporters of the theory can only mutely regret that 
they have no chance of making so distinguished a convert ; 
but, when he quotes the words I have copied as an argument 
against the theory, it seems desirable to call attention to the 
fact that those remarks were really aimed at something else, 
Prof. Armstrong attacks the whole idea of charged ions, and 
quotes Maxwell to show that it is only a provisional hypothesis. 
Of course the convection theory of an electrolytic current is merely 
a convenient way of stating observed facts, such as those grouped 
together under Faraday’s law. It seems clear that the path ot 
the energy used by the current lies inthe neighbouring dielectric, 
just as is the case with a metallic circuit, and it is probable that 
the real part played by the mobility of the ions is to allow the 
slipping to go on, by means of which can alone occur that trans- 
formation into heat of the energy of the polarised dielectric 
which we call an electric current. But, whatever the true nature 
of the process, it is certain that the passage of a definite quantity 
of electricity through a solution is always associated with the 
decomposition of a definite quantity of the electrolyte, and, 
therefore, with the passage in opposite directions of a definite 
quantity of the opposite products of decomposition—call them 
ions or what you will. To this extent, then, we are free from 
hypotheses, and this is ground enough on which to found the 
whole theory of ionic migration and its consequence, the dis- 
sociation of the ions from each other. 
Prof. Armstrong complains of the ease with which the dis- 
sociation theory can be reconciled with new views as they are 
developed, and can be made to explain new phenomena as they 
are discovered. It seems to me that this adaptability goes to 
show that the fundamental idea of the theory is in accordance 
with the facts of the case. 
I have no desire to defend the moral character of the ions 
when they have freed themselves from the bonds of that union 
which Prof. Armstrong seems to think should possess the sanc- 
tity of the marriage tie. I fear it is very likely that they do 
form those connections which he has condemned in such very 
plain language. Indeed, things may be even worse than he 
suggests. The ions may actually pass from one molecule of the 
solvent to another, carrying their charges with them, till con- 
tact with an electrode ends their career of infamy. Consider- 
able mental agility is needed to follow a// the metaphors which 
Prof. Armstrong crowds into a single sentence, but, at any 
rate, it is plain that this freedom of life clears the ions from 
the accusations of bearing ‘‘ water molecules on their backs,” 
and of being ‘‘ chained to them like galley-slaves.” 
With regard to the velocities of the ions, I cannot quite see 
the reason for Prof. Armstrong’s belief that they are really 
higher than is generally supposed. Even on the purely gaseous. 
theory of solution, which Prof. Armstrong here uses to support 
his argument, the dissolved ions must have very short free 
paths, and, although the velocity with which they vibrate back- 
wards and forwards from one solvent molecule to another may 
