502 
substance. The ions can be indeed separated from each other, but 
not to continue as themselves, since in the act of separating they 
form ordinary substances, either by uniting with other ions, or 
by two molecules of ion becoming one molecule of substance, 
In the former way of separation the ions of two salts interact on 
mixing their solutions; in the other way, the ions become sub- 
stances when their solution is placed in a galvanic circuit. In 
this moce of separation—by electrolysis, that is—the substances 
corresponding with the two ions, or else secondary products of 
their change, are produced, the one substance at the kathode and 
the other at the anode, while the solution away from the 
electrodes, but between them, remains for the time unaltered in 
composition. Along with this there occurs in many cases a 
phenomenon first recorded by Daniell, and afterwards investi- 
gated by Hittorf with such beautiful results. This consists in a 
greater fall taking place in the concentration of the salt solution 
close to one electrode than in the concentration of that close to 
the other, as though the ions were hydrate compounds, and that 
the one ion was a higher hydrate than the other. Until we know 
more of the nature of the ions themselves this phenomenon is 
most conveniently quantified on the hypothesis that the ions 
travel as molecular particles, but the discussion of this hypothesis 
is beside my present purpose. 
The phenomena of ionisation or, in other words, the particular 
properties of dilute solutions of salts, belong evidently to a 
change unlike all other chemical changes. It is a polarised 
chemical change, in which the equivalent and complemental 
products of the interaction appear apart and at remote surfaces 
of the mass of decomposing salt solution. Two points which 
call for notice in connection with my present subject are that an 
ion is one of a pair of quantities commensurate with the quantity 
of the salt itself that is or would be in interaction ; and that it 
is molecular in character and therefore to be regarded as a 
relative and wholly variable quantity. 
Dalton’s atoms were both the atoms and the molecules of 
present-day chemistry, but much more the latter than the 
former. Although the chemical atom can now be no more than 
a dependency of the molecule, it is commonly set up as the 
starting-point in chemical theory, and as having an independent 
existence as a quantity of the substance, while the molecule is 
represented as being a conjugation of atoms. But there cannot 
be two standards in reference to the same thing, and in molecular 
chemistry the atom must give way. As I have already had 
occasion to point out, the atom is of the radical, the molecule 
is of the substance. 
The four radicals of a double decomposition are equal and 
chemically complementary. These chemically equal quantities 
of such radicals are atoms. The quantities of all other radicals 
are also atoms, but only those of proximate radicals, those of a~° 
single interaction, are equal. Similarly, the quantities of the 
four substances of a single interaction are all equal and are mole- | 
cules, but the quantities of substances are not equal in other 
interactions. These others are treated as the simultaneous oc- 
currence of two or more single interactions, which they can 
always be represented and sometimes demonstrated to be. 
Calcium hydroxide and hydrogen sulphide give calcium hydro- 
sulphide and water by two single interactions together, which 
in this case can be easily distinguished, since the calcium 
hydroxide will also interact with only half as much hydrogen 
sulphide to form the insoluble crystalline calcium hydroxyhydro- 
sulphide and half as much water as betore ; this calcium salt will 
then interact with as much more hydrogen sulphide as went to 
form it, and produce the very soluble crystalline calcium hydro- 
sulphide, Or the calcium hydrosulphide and as much calcium 
hydroxide as yielded it will readily interact to form twice as much 
as the first-obtained quantity of calcium hydroxyhydrosulphide. 
Thirdly, the calcium hydrosulphide and half as much water as 
was formed with it from calcium hydroxide readily interact to 
produce calcium hydroxyhydrosulphide, and half as much 
hydrogen sulphide as was needed to form the hydrosulphide. 
Therefore, and on other grounds, we say and know that one 
molecule of calcium hydroxide and two molecules of hydrogen 
sulphide give one molecule of calcium hydrosulphide and two 
molecules of water. This is, of course, only the law of multiple 
proportions introduced into chemical interactions. The ex- 
pression ‘‘two or more molecules of a substance”’ has a meaning 
only as indicating the number of simultaneous or successive 
single interactions which have led to the conversion of certain 
substances into others. 
Now a similar but complementary state of things meets us in 
NO. 1716, vot. 66] 
NATURE 
[SEPTEMBER 18, 1902 
the case of radicals. Instead of the coeflicients of molecules, 
necessitated by having to consider many chemical changes as 
being cases of two or more single interactions occurring together, 
there are the valency coefficients of the polyvalent radicals, called 
out also by such a compound interaction. Thus, in the above 
case, whilst the single interaction between hydrogen sulphide and 
calcium hydroxide shows calciumhydroxyl as one of the radicals, 
the succeeding interaction between the calcium hydroxyhydro- 
sulphide and more hydrogen sulphide shows the radical calcium- 
hydrosulphury], and the common part of these two radicals is the 
bivalent radical, calcium. It will be evident that to give the 
atom of the calcium radical as bivalent is a statement reciprocal 
or complementary to that of giving two molecules of hydrogen 
sulphide as interacting with one of calcium hydroxide. Chemical 
equality remains still the measure of the atom, but that, in com- 
plex changes, whereas the number of molecules of one substance 
marks the number of single interactions, the valency number of 
the atom marks the same thing for the radical. Itisa matter of 
valency, and not otherwise a matter of the atom. The radical 
calcium is never actively bivalent in a single interaction ; in other 
words, it is never equal to two atoms of hydrogen. As a simple 
radical it does not take part in such an interaction ; but it does 
do so as a radical of radicals, such as calciumhydroxyl and 
calciumhydrosulphuryl, and then has the same measure as—is 
equal in exchange to—the atom of hydrogen, though carrying 
with it of necessity other radicals, a thing the hydrogen radical 
never does or can do. To take another example ; when 
acetamide is formed from acetic acid, the nitrogen of the 
amidogen and the oxygen of the hydroxy] are equal in exchange, 
but because of their valencies the one carries with it two atoms 
and the other one atom of hydrogen. This is no matter of 
merely academic contention, for upon its recognition rests the 
doctrine of valency itself. 
The quantity of the radical is the only proper and sufficient 
definition of the atom, whether the radical be that of a single 
interaction, or a radical of radicals, that is, a polyvalent radical. 
The atom is, therefore, the quantified power of a substance, as 
the compound of the radical, to produce other compounds of the 
radical, including its compound with itself, where that is 
possible. As with the molecule of a substance, so with the 
atom of the radical, it is of no fixed magnitude, and may weigh 
a kilogram just as well as only a milligram or something much 
less. Being a relative quantity and nothing by itself, of its 
indivisibility there is nothing to be said outside its definition ; 
whilst, as to its being the smallest relative quantity inter- 
changing in an interaction, it had only thus to be defined when 
there was uncertainty as to the molecule and the single 
interaction. 
It has been impossible for me to discuss the nature of 
the radical and the atom without referring to valency, but 
it is itself a subject of such importance as to need special 
consideration. It does not seem right to me to say even the 
little I can say about valency without naming with the respect 
they deserve from us the distinguished chemists who laid the 
foundations of the doctrine and developed it: Williamson, 
Odling, Wurtz, Edward Frankland and Kekulé. I had the 
good fortune to be in the same laboratory as, and then intimate 
with, Kekulé when, in 1854, he was working out the bivalency 
of sulphur and oxygen by his investigation of thioacetic acid, 
some lime, that is, before he had thought out the benzene ring 
and the valency of carbon. 
Only when, as is usual, propositions are made in which a 
separate and independent existence, with valency asa property, 
is imputed to a radical does the question, as to what valency is, 
present any difficulty. Approaching it from the side of the mole- 
cule and of double decomposition, and therefore from the 
experimental side instead of from that of the radical itself, as is 
customary, valency presents itself as being the number of single 
interactions necessary in order to have a certain radical occur, 
first as that of one substance, and then as that of another which 
has no other radical in common with the first substance. That 
ammonia possesses one atom of the radical nitrogen and three 
atoms of the radical hydrogen, and that the nitrogen radical is 
tervalent and the hydrogen radical univalent, are statements 
mutually based upon facts such as the following. Potassium 
nitrilosulphate, which contains nitrogen but no hydrogen, is 
converted by water in a sharply defined single interaction, into 
potassium hydrogen sulphate, and into potassium imidosulphate, 
a substance which contains all the nitrogen along now with 
hydrogen. This salt passes, also sharply and by a single inter- 
