Sept. 17, 1885 | 
NATURE 
469 
believe that we shall be able to state the exact conditions under 
which chemical change will take place until a satisfactory 
solution has been found. 
F. ‘Kohlrausch (Poge. Ann. 1876, 159, 233) has shown that, 
on adding sulphuric acid to water, the electric conductivity in- 
creases very rapidly until when about 30 per cent. of acid is 
present a maximum (6,914) is attained ; conductivity then dim- 
inishes almost as rapidly, and a minimum (913) is reached when 
the concentration corresponds with that of a monohydrate 
(H,SO,,0OH,) ; from this point conductivity increases some- 
what (to 1,031 at 92°r per cent. H,SO,), and then again 
falls, and is probably zero for the pure acid ; on adding sulphuric 
anhydride to the acid conductivity again increases. Solutions of 
other acids and of a number of salts—chiefly deliquescent and 
very soluble salts—also exhibit maximum conductivity at parti- 
cular degrees of concentration. In no other case has the ex- 
istence of two maxima, such as are observed in solutions of 
sulphuric acid, been established ; but probably this is because the 
experiments either have not been, or cannot well be, carried out 
with pure substances or very concentrated solutions. Solutions 
of less soluble salts increase in conductivity as the amount of salt 
dissolved increases. 
Kohlrausch has suggested, as an explanation of the influence 
of the ‘‘solvent ” on the conductivity of an ‘‘ electrolyte,” that 
in a solution the ions which are being transferred electrolytically 
come less frequently into collision than would be the case in the 
pure substance. There is therefore less opportunity for the for- 
mation of new molecules, and the ions are able to travel farther 
before entering into combination. 
Regarding the question from a chemist’s point of view, how- 
ever, I cannot help thinking that this explanation is scarcely 
satisfactory or sufficient ; but I cannot resist the feeling that the 
production of electrolytically conducting solutions from dielectrics 
is in some measure dependent upon the occurrence of chemical 
action. If the composition of the solutions of maximum con- 
ductivity be calculated,’ it will be seen that they contain but a 
limited number of water molecules ; thus the solution of sulphuric 
acid of maximum conductivity (at 18°) contains 30°4 per cent. of 
acid, and therefore has the composition H,SO4:12°4 H,O 
(approximately); for nitric acid the ratio is 1:8; for acetic 
acid it is about 1:17. Now, it is highly remarkable that the 
solutions of maximum: electric conductivity are also very nearly 
those in the formation of which nearly the maximum amount of 
heat is developed ; this will at once be obvious on comparison of 
the curves given by Thomsen (‘‘Thermochemische Unter- 
suchungen,” vol. iii.) and by Kohlrausch. In the chemist’s 
experience, the point of maximum heat development is usually 
near to the point of maximum chemical change, and I think, 
therefore, that we are justified in concluding that, even if 
electrical ‘conductivity be not a maximum at a particular con- 
centration on account of the presence of a particular hydrate 
(belonging to the class of molecular aggregates) in maximum 
amount, at all events the ‘structure ” of the system is especially 
favourable, and the ‘‘ chemical influence ”’ exerted by the one set 
of molecules upon the other is at a maximum at the point of 
maximum conductivity. The fact that the amount of sulphuric 
acid required to form a solution of maximum conductivity in- 
creases with temperature— 
C) 
Temp. fo) 
Per cent. 
TOM 20M ESOmm AO sms Ou (GOn—N7Oz 
Sore iol) Shelly Beaks GBS) SUE VISES GZ! 
and also the fact that the maxima and minima of conductivity 
tend to become obliterated with rise of temperature (Kohlrausch), 
are both in accordance with the view that conductivity is in some 
way dependent upon chemical composition, as the effect of rise 
of temperature would be to cause the dissociation of hydrates 
such as I have referred to. The increase in conductivity of 
aqueous solutions with rise of temperature would appear to be 
against the view here put forward ; but it is probable that this 
1 Formula Formula Percent.in Compositionin Conductivity 
‘ weight solution of approximate mol. 
max. cond, ratios 
HNO, 63 29°7 ie. 7330 
HCl 30°4 18°3 biG Ae) 7174 
H,SO, 98 30°4 1:12°4 6914 
H3PO, 98 40'8 1-10) 1962 
C,H,O, 60 16°6 mga 152 
KOH 56 28°1 Ee 5995 
NaOH 49 152 I: 12°7 3276 
may be largely due to diminution in viscosity and increase in the 
rate of diffusion. 
Our knowledge of the binary metallic compounds, which are 
generally admitted to be electrolytes er se, also affords evidence, 
[ think, of an intimate relation between chemical constitution 
and *‘electrolysability.” It has been pointed out (comp. L. 
Meyer, “‘ Theorien d. mod. Chemie,” 4th ed. p. 554) that, 
whereas all the metallic chlorides and analogous compounds 
which cannot be electrolysed are easily-volatile bodies, the 
electrolysable metallic chlorides, &c., are fusible only at high 
temperatures. A careful discussion of the various known cases 
does not, however, justify the conclusion that decomposition 
takes place, or not, according as the temperature at which the 
body assumes the liquid state—-and at which, therefore, there is 
full opportunity given for electrolysis to take place—is high or low, 
especially as recent observations show that electrolysis may take 
place prior to fusion. But it is especially noteworthy that many 
of the chlorides, &c., which are electrolytes undoubtedly contain 
more than a single atom of metal in their molecules ; indeed, 
after careful consideration of the evidence, I am inclined to go 
so far as to put forward the hypothesis ¢hat among metallic com- 
pounds only those are electrolytes which contain more than a single 
atom of metal in their molecules. No difficulty will be felt in 
granting this of cuprous and stannous chlorides, and even of 
cadmium, lead, silver, and zinc chlorides; but opinions will 
differ as regards the metals of the alkalies and the alkaline 
earths.’ Assuming the constitution of metallic electrolytes to be 
such as I have suggested it is not improbable that on electrolysis 
a part only of the metal is determined to the one pole, the 
remainder being transferred along with the negative radical to 
the opposite pole. Hittorf, indeed, has already put forward this 
view in explanation of the remarkable results he obtained 
on determining the extent of transfer of the ions in aqueous 
and alcoholic solutions of the chloride and iodide of cadmium 
and zinc. 
Again, an argument in favour of a connection between chemical 
constitution and electrical conductivity is the fact that carbon, 
sulphur, selenium and fhosphorus each exist in conducting and 
non-conducting modifications, as it can scarcely be doubted that 
the so-called allotropic modifications of these elements are 
differently constituted. 
It appears, as I have already said, to be the current belief that 
when aqueous solutions are submitted to electrolysis, as a rule 
the dissolved substance, and not the water, is the actual electro- 
lyte. Without reference to the question I have raised as to 
the constitution of an electrolyte, it appears at least doubtful 
whether this view can be justified by appeal to known facts ; at 
all events, I have failed to find satisfactory evidence that such is 
the case. Moreover, as sulphuric anhydride dissolves in water 
with considerable development of heat, it would appear that more 
work has to be done to separate hydrogen from sulphuric acid 
than to separate it from water ; on this account we might expect 
that the water rather than the acid would be decomposed. Are not 
perhaps both affected according to the proportions in which they 
are present? The marked variation in the extent to which the 
negative ion is transferred to the positive pole, as observed by 
Hittorf, when solutions of different degrees of concentration are 
electrolysed, would appear to support this view. The difference in 
the products, according as dilute or very concentrated solutions of 
sulphuric acid are used, may also be cited as an argument that the 
chemical changes effected vary with the concentration ; but, on the 
other hand, it is quite possible that the observed differences may 
result from the occurrence of purely secondary changes. Ostwald 
has recently put forward the view that one or move of the hydrogen 
atoms of certain acids are split off according to the concentration 
of the solution. 
I call attention to this because I conceive that it has a most 
1 We may regard as evidence in support of this explanation the fact that 
neither beryllium chloride, which fuses at 600°, nor mercuric chloride, is an 
electrolyte, as both of these, at temperatures not far removed from their 
boiling-points, exhibit the simplest possible molecular composition. It should 
be pointed out, however, that Nilson and Patterson found it possible to 
determine the density of beryllium and chloride gas at a temperature 
too’—r50° below the melting-point found by Carnelly; but they were not 
able to say that fusion took place. Clarke’s recent interesting observations 
on mercuric chloride and iodide do not, I think, suffice to prove that these 
compounds are electrolytes ; it is more than probable that electrolysis is 
preceded by the formation of mercurous compounds. Even an aqueous 
solution of mercuric chloride does not conduct appreciably better than water 
(Buff). I should parhaps add that the mere presence of more than a single 
atom of metal in the molecule does not, I believe, alone constitute the 
compound an electrolyte ; much depends probably both on the nature of the 
metal and on the structure of the molecule. 
