468 
should scarcely now be so ignorant as we are of the conditions 
of chemical change. 
The questions—What is Electrolysis? What is an Electro- 
lyte? are all-important to the chemist, if my contention be 
accepted. Moreover, the consideration of chemical action from 
this point of view almost of necessity obliges us also to consider 
what it is that constitutes chemical affinity. I will not presume 
to offer any opinion on this subject ; but I would recall atten- 
tion to the prominence which so great an authority as Helm- 
holtz gave in the last Faraday Lecture (Chem. Soc. Zrans., 
1881, 277) to the view held by Faraday, and which is so 
efinitely stated in a passage in his ‘‘ Experimental Researches” 1 
(series viii. 918, also 850 and 869). 
Helmholtz used the words: ‘‘I think the facts leave no doubt 
that the very mightiest among the chemical forces are of electric 
origin. The atoms cling to their electric charges, and opposite 
electric charges cling to each other; but I do not suppose that 
other molecular forces are excluded, working directly from atom 
to atom.” In the passages which immediately follow, this 
physicist then makes several statements of extreme importance, 
which directly bear upon the subject I desire to discuss, and 
which, therefore, I quote.” 
The interpretation of Faraday’s law of electrolysis, which 
Helmholtz has brought under the notice of chemists, is of the 
most definite and far-reaching character. Does it, however, at 
all events in the form in which he has put it forward, accord 
is acting upon the water as the reciprocals of each other. In both parts we 
have the two conditions, zzseparable in such bodies as these, namely, the 
passing ofa current and decomposition ; avd this ts as true of the cells in 
the battery as of the water-cell; for no voltaic battery has as yet been con- 
structed in which the chemical action is only that of combination: 
aecontposition is always included, and is, I believe, an essential chemical 
part 
““ But the difference in the two parts of the connected battery—that is, the 
decomposition or acting cells—is simply this: in the former we urge the 
current through, but it, apparently of necessity, is accompanied by decom- 
position; in the latter we cause decompositions by ordinary chemical 
actions (which are, however, themselves electrical), and, as a consequence, 
have the electrical current ; and as the decomposition dependent upon the 
current is definite in the former case, so is the current associated with the 
decomposition also definite in the latter.” 
1 “All the facts show us that that power commonly called chemical 
affinity can be communicated to a distance through the metals and certain 
forms of carbon ; that the electric current is only another form of the forces 
of chemical affinity ; that its power is in proportion to the chemical affinities 
producing it ; that when it is deficient in force it may be helped by calling in 
chemical aid, the want in the former being made up by an equivalent of the 
latter; that, in other words, the forces termed chemical affinity and 
electricity are one and the same.” 
Several of our leading chemists have lately begun to distinguish two 
classes of compounds—viz. molecular aggregates and typical compounds, the 
latter being united by atomic affinities, the former not. Electrolytes belong 
to the latter class. If we conclude from the facts that every unit of affinity 
is charged with one equivalent, either of positive or of negative electricity, 
they can form compounds, being electrically neutral, only if every unit 
charged positively unites under the influence of a mighty electric attraction 
with another unit charged negatively. You see that this ought to produce 
compounds in which every unit of affinity of every atom is connected with 
one, and only one, other unit of another atom. This, as you will see imme- 
diately, is the modern chemical theory of quantivalence, comprising all the 
saturated compounds. The fact that even elementary substances, with few 
exceptions, have molecules composed of two atoms makes it probable that 
even in these cases electric neutralisation is produced by the combination of 
two atoms, each charged with its full electric equivalent, not by neutralisa- 
tion of every single unit of affinity. Unsaturated compounds with an even 
number of unconnected units of affinity offer no objection to such an 
hypothesis: they may be charged with equal equivalents of opposite elec- 
tricity. Unsaturated compounds with one unconnected unit, existing only 
at high temperatures, may be explained as dissociated by intense molecular 
motion of heat, in spite of their electric attractions. But there remains one 
single instance of a compound which, according to the law of Avogadro, 
must be considered as unsaturated even at the lowest temperature—namely, 
nitric oxide (NO), a substance offering several very uncommon peculiarities, 
the behaviour of which will be perhaps explained by future researches.”’ 
The popular mistake is here made of assuming that elementary substances, 
with few exceptions, have molecules composed of twoatoms. We now know 
considerably over seventy elements, but of these the molecular weights in 
the gaseous state of only thirteen have been satisfactorily determined. 
The gaseous elements hydrogen, oxygen, nitrogen and chlorine, and also 
bromine, iodine and tellurium, have diatomic molecules ; phosphorus and 
arsenic have tetratomic molecules; those of sulphur are hexatomic, and 
selenium molecules are probably of similar constitution, but more readily 
broken down than those of sulphur ; lastly, cadmium and mercury molecules 
are monatomic _ It is more than probable that carbon, and also silicon and 
boron, form highly complex molecules. Of the remaining undetermined 
elements, the greater number are metals, and it is not unreasonable to 
assume that many of these will be found to resemble cadmium and mercury 
in molecular composition. It is clear, however, that at present we have 
no right to say that the elementary molecules are, as a rule, diatomic. 
It would assist in removing this error if chemists would consistently place 
after the symbol the numeral indicating the “‘ atomicity ” of the elementary 
molecule—thus, Hg,, Cd,, O.; and if in all cases when a numeral is absent, 
or is placed defore the symbol, it were understood that advisedly no indica- 
tion of the molecular state is afforded. 
NATORE 
[ Sept. 17, 1885 
sufficiently with the facts as these present themselves to the 
chemist’s mind? All will recognise that the chemical changes 
effected by a current in a series of electrolytic cells are equiva- 
lent to those which take place within the voltaic cells wherein 
the current is generated ; but in neither case is the action of a 
simple character: in both a variety of chemical changes takes 
place, the precise character of which is but imperfectly under- 
stood, and we are unable to assign numerical values, either in 
terms of heat or electrical units, to most of the sefarate changes. 
Moreover, many compounds are not electrolytes, while others 
which are regarded by the chemist as their analogues are very 
readily decomposed by a current of low E.M.F., although no 
great difference is to be observed in their ‘‘ heats of formation ;” 
liquid hydrogen chloride on the one band, and fused silver 
chloride on the other, may be cited as examples. Again, how 
are we to interpret on this theory such changes as that involved 
in the conversion of stannic into stannous chloride? The 
former, I suppose, is to be regarded as consisting of an atom of 
quadrivalent tin charged with four units of, say, positive elec- 
tricity, and of four atoms of univalent chlorine, each carrying a 
unit charge of negative electricity ; on withdrawal of two of the 
chlorine atoms, the residual SnCl, will have two free unit 
charges of positive electricity. We know that when the tem- 
perature is sufficiently lowered two such residues unite, forming 
Sn,Cl,, and it is not improbable that crystalline stannous chloride 
represents a still laterstage of condensation. Is this compatible 
with the theory? That cases of this kind are contemplated 
would appear from the reference to ‘‘unsaturated compounds 
with an even number of unconnected units of affinity,’ which 
we are told may be charged with equal equivalents of opposite 
electricity ; and also from the allusion to the existence of mole- 
cules of elementary substances composed of two atoms. It is 
more than probable that these anomalies would disappear on 
fuller statement of his views by the author of the theory: I have 
ventured to call attention to them in the hope of eliciting such 
statement. 
Helmholtz tells us that electrolytes belong to the class of 
typical compounds, the constituents of which are united by 
“atomic affinities,” not to the class of ‘‘ molecular aggregates.” 
Is this the fact? Before chemists can accept this conclusion 
many difficulties must be removed which appear to surround the 
question. In the first place, it is in the highest degree remark- 
able that, with the one single exception of /iguefied ammonia, 
no known binary hydride is in the liquid state an electrolyte: 
liquid hydrogen chloride, bromide and iodide, for example, with- 
standing an E.M.F, of over 8,000 volts (8,040 De la Rue cells : 
Bleekrode). Water, again, according to Kohlrausch’s most 
recent determinations, has an almost infinite resistance. Yet a 
mixture of hydrogen chloride and water readily conducts, and is 
electrolysed ; an aqueous solution of sulphuric acid behaves 
similarly, although the acid itself has a very high resistance.! 
Very many similar examples might be quoted, but it is well 
known that aqueous solutions generally conduct more or less 
perfectly, and are electrolysed.? 
The current belief among physicists would appear to be that 
the dissolved electrolyte—the acid or the salt—is almost exclus- 
ively primarily decomposed (Wiedemann, ‘‘ Elektricitat,” 1883, 
ii. 924). We are commonly told that sulphuric acid is added to 
water to make it conduct, but the chemist desires to know why 
the solution becomes conducting. It may be that in all cases 
the ‘‘ typical compound ” is the actual electrolyte—z.e, the body 
decomposed by the electric current—éwt the action only takes place 
when the typical compounds are conjoined and form the molecular 
ageregate, for it is an undoubted fact that HCl and H,SO, 
dissolve in water, forming ‘‘ hydrates.” This production of an 
“electrolytical system” from dielectrics is, I venture to think, 
the important question for chemists to consider. I do not 
1 It is more than probable that the most nearly pure sulphuric acid which 
can be obtained is not homogeneous, but is at least a mixture of H,SO4, 
H,S,07 and “‘hydrated compounds” in proportions depending on the 
temperature, and hence that (pure) sulphuric acid, H.SOy, like water, would 
behave as a dielectric. : 5 
? On the other hand, it is remarkable that, whereas liquefied ammonia may 
be electrolysed, an aqueous solution of ammonia is a most imperfect con- 
ductor (Faraday, F. Kohlrausch), although solutions of ammonium salts 
compare favourably in conductivity with corresponding sodium and potassium 
salts. This fact serves somewhat to allay the suspicion that Bleekrode did 
not take sufficient precautions to dry the ammonia ; but his result cannot, I 
think, be accepted aS final, on account of the relatively high E.M.F. 
required, and the repetition of the experiment with every precaution to 
ensure purity of the gas is most important. Faraday regarded the decom- 
position of ammonia on electrolysis of its solution as merely the result of 
secondary action. 
