TRANSACTIONS OF SECTION B. 955- 
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 sufficientiy 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 equivalent 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 understood, and 
we are unable to assign numerical values, either in terms of heat or electrical 
units, to most of the separate 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 hand, and fused silver chloride on the other, may he 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 electricity, 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 temperature is sufficiently lowered two such residues unite, forming 
Sn,Cl,, and it is not improbable that crystalline stannous chloride represents a still 
later stage of condensation. Is this compatible with the theory? That cases of 
this kind are contemplated would appear from the reference to ‘ unsaturated com- 
pounds 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 molecules 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 ciass of typical compounds, 
the constituents of which are united by ‘atomic affinities,’ not to the class of 
charged with its full electric equivalent, not by neutralisation 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 electricity. 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 iaw 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 two atoms. We now know considerably over seventy elements, but of 
these the molecular weights in the gaseous state of only thirteen have been satis- 
factorily determined. The gaseous elements hydrogen, oxygen, nitrogen and chlo- 
rine, 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 before the symbol, it were 
understood that advisedly no indication of the molecular state is afforded. 
5 
