JANUARY 7, 1597 | 
NATURE 
223 
LETTERS TO THE EDITOR. 
[Vhe Editor does not hold himself responsible for opinions ex- 
pressed by his correspondents. Neither can he undertake 
to return, or to correspond with the writers of, rejected 
manuscripts intended for this or any other part of NATURE. 
No notice is taken of anonymous communications.) 
On a New Law Connecting the Periods of 
Molecular Vibrations. 
In the current number of NATURE you were good enough to 
print a short article by me, announcing a discovery which I be- 
lieved was new. My attention has now been drawn to the fact 
that it was published a few months ago by Rydberg (Weed. 
Ann., vol. lviii. p. 674), to whom the honour of the discovery 
therefore belongs. |My excuse for being unacquainted with 
Rydberg’s paper must be found in a prolonged absence from 
home last summer, and the large amount of unread scientific 
literature which I consequently found on my return home. 
There is, moreover, nothing in the title of Rydberg’s paper 
which would indicate the important nature of its contents. If by 
writing to you on the subject I have drawn the attention of 
physicists to what I consider the most important fact yet brought 
to light concerning molecular vibrations, my article will have 
served some good purpose. ARTHUR SCHUSTER. 
The Atheneum, Pall Mall, S.W., January 2. 
The Pound as a Force. 
A very few words are necessary from me in answer to Prof. 
Perry’s letter on page 177. First and foremost (though referring 
to the latter part of his letter, not to the cow and bridge por- 
tion), if any sentence in my previous communication can have 
led any one to imagine that I consider Prof. Perry anything but 
a most admirable teacher of his own subject, that sentence must 
have been villainously expressed. Secondly, when [I said that 
engineers had mostly to deal in their calculations with bodies 
either at rest or in uniform motion, I thought I was speaking in 
the sense of Prof. Perry’s original article (he said the same thing 
himself near the top of column 1, page 50), and that I should 
have his concurrence: I would not for a moment argue such a 
point with him. IfI had thought it necessary to be cautious I 
would have used the word ‘‘suggest”’ instead of the word 
**tell” in my sentence about acceleration : to the idea in which 
however I still respectfully adhere. And in general I adhere toall 
the matfer of my last communication, though with full deference 
to his criticism on the manner of it. Thirdly, I cannot 
remember that I have ever specially ‘‘advocated” the 
poundal. I have never much liked it, but it is useful as 
a stepping-stone to higher things, in a way that the familiar 
pound-weight is not. Fourthly, I agree with Fitzgerald 
that Newton’s second law furnishes by no means the only 
measure for quantity of matter (chemical equivalence also 
furnishes a measure), but inertia is the fundamental property 
and measure for dynamical purposes.  Fifthly, we do not 
*fassume”’ that inertia is proportional to weight ; we verify it 
within certain limits of error by dropping bodies (like Galileo), 
or (like Newton) within narrower limits by swinging pendu- 
lums: essentially the same process. Sixthly, I do not, alas, 
find it at all easy to give full marks to a student for his answer to 
such a question as ‘* What is Ohm’s law?” ; and, although I 
cannot plead guilty to the accusation of having spoke disrespect- 
fully either of Gravity or of Engineers, I do find that occasionally 
the treatment of the former by the latter leaves something to be 
desired in point of clearness ; the occasional educational remarks 
of the periodical called Zhe Zngineer, for instance, seem fairly 
representative of a large and influential class. And lastly, 
although a remark immediately following his citation of a 
familiar electrical equation leads me to think that Prof. Perry 
still misses the chief point of my letter, yet there are quantities 
of things in the present correspondence on which we agree ; and 
chief among them is the profound conviction we share that there 
is a crying need for reform in our whole system of secondary 
education. OLIVER J. LopGE. 
Liverpool, December 27, 1896. 
The Theory of Dissociation into Ions. 
THE numerical agreement obtained when certain properties 
of solutions are interpreted on the theories of osmotic pressure 
and ionic dissociation is undoubtedly very striking, and it is, 
NO. 1419, VOL. 55] 
consequently, not very surprising that these theories have ob- 
tained such a ready acceptance. Whatever may be our opinions 
as to the validity of the theories, and even of the harm which 
has been done by pressing them too far, we cannot but recognise 
that they have been the origin of much good work on a condi- 
tion of matter which is, at the same time, one of the most 
obscure and one of the most important, both from the physicist’s 
and chemist’s point of view. But, however convenient such 
theories may be as working hypotheses, their advocates should 
not have forgotten that they depend solely on the numerical 
relations alluded to, and that something more than this is 
required before such hypotheses can be raised to the level of 
acceptable theories, and far more before they should be held up 
as an indispensable article of faith, which unless a chemist 
believe he cannot be saved. 
For a theory to be acceptable it should, at the very least, be 
reasonably probable, and should not violate any fundamental 
and well-established facts; it should stand the test of any 
apparently crucial experiments brought forward to settle between 
it and its rivals, and, I think we may add, it should give some 
explanation, not simply of the behaviour of matter in the condi- 
tion in question, but also of why matter ever assumes such a 
condition. 
The theories cf osmotic pressure and ionic dissociation, I 
believe, have not done this. Even if we can accept as probable 
the view that atoms united so firmly together, as we have every 
reason to believe are those of, say, chlorine and hydrogen, will 
fly affrighted from each other at the mere approach of a few 
water molecules, which are represented as being more or less 
inert and destitute of any strong attraction for the dissociated 
atoms ; even if we can imagine that these atoms, so strongly 
charged with electricity of opposite signs, can meander about 
in the liquid, with a supreme disdain for their former associates 
and the attractive charges which they carry; even if we can 
reconcile this indifference with the behaviour of these very atoms 
to a similar electric charge on other similar companionable 
atoms, when these latter happen to be agglomerated into the 
form of an electrode ; even if we find no difficulties in all this, 
still we must admit that the theories in question afford no 
explanation whatever why a substance should dissolve at all, and 
they can, therefore, hardly be accepted as a sufficient explana- 
tion of solutions. We cannot treat Nernst’s statement that a 
substance goes into solution because it has a ‘* solution pressure ” 
seriously. and, in cases where the dissolved substance is known 
to form hydrates, the view that an excess of water will decom- 
pose these hydrates, and free the substance entirely from its 
union with water, without the formation of any other com- 
pounds, is quite opposed to our knowledge of the action of 
mass in chemical changes. 
Nor can we ignore the thermal difficulties in which the theory 
of dissociation lands us; for if, to satisfy the facts of the case, 
we admit that dissociation is accompanied by a large evolution 
of heat, we must suppose, either, that the evolution which 
accompanies the reverse action when the water is absent 
(e.g. Hy + Cly = 2HCI, gases), is due to heat being evolved 
by the dissociation of molecules of elements into their atoms, or, 
as has been asserted, that the atoms of the dissolving electrolyte 
evolve heat by combining with their electric charges, a novel 
method of evolving heat, which should long ago have made the 
fortunes of the discoverers, especially as the charges with which 
the atoms combine come into existence of their own accord, and 
without the expenditure of any external energy. : 
Turning to the ‘‘crucial” experiments suggested, we do not 
find the results to be any more satisfactory from the point ot 
view of the theory. We have on the one side two experiments 
heralded in by Prof. Ostwald with great flourish of trumpets ; 
the ‘‘ imaginary *’ experiment already quoted by Dr. Herroun, 
in which an ultra-microscopical trace of liquid is electrolysed by 
an electrostatic discharge, and the ‘‘arm-chair” experiment of 
‘chemical action at a distance,” the results of both of which 
might have been predicted, as I have shown elsewhere, by any 
one possessing an elementary knowledge of electricity, long 
before the dissociation theory was dreamt of. 
On the other side we have two experiments, which would 
seem to be conclusive, but which the dissociationists have 
hitherto thought fit to ignore. 
Osmotic pressure, they hold, is due to the quasi-gascous 
pressure of the solvent and dissolved substance acting on a 
diaphragm, which, being permeable to the solvent only, renders 
the pressure of the dissulved substance inoperative, and hence 
