Dr. Odling delivered a lecture on the above before the Chemical Society,
London, February 2, 1882.
The lecturer said that it had been found useful to occasionally bring
forward various points of chemical doctrine, on which there were
differences of opinion, to be discussed by the society. On this occasion
he wished not so much to demonstrate certain conclusions, or to make a
declaration of his opinions, as to invite discussion and a thoughtful
consideration of questions of importance to chemists. Originally three
questions were proposed: First, Is there any satisfactory evidence
deducible of the existence of two distinct forms of chemical combination
(atomic and molecular)? Second, Is the determination of the vapor
density of a body alone sufficient to determine the weight of the
chemical molecule? Third, In the case of an element forming two or more
distinct series of compounds, e.g., ferrous and ferric salts, is the
transition from one series to another necessarily connected with the
addition or subtraction of an even number of hydrogenoid atoms? He
would, however, limit himself to the first of these questions; at the
same time the three questions were so closely associated with one
another that in discussing the first it was difficult to know where to
begin. The answer to this question (Is there any satisfactory
evidence deducible of the existence of two distinct forms of chemical
combination?) depends materially on the view we take of the property
called in text-books valency or atomicity; and before discussing the
question it is important to have a clear idea of what these words
valency and atomicity really mean. It is necessary, too, to start with
some propositions which must be taken for granted. These propositions
are: First, that in all chemical changes, those kinds of matter which we
commonly call elementary, do not suffer decomposition. Second, That the
atomic weights of the elements as received are correct, i.e., that they
do really express with great exactitude the relative weights of the
atoms of the individual elements. If we accept these two propositions,
it follows that hydrogen can be replaced atom for atom by other elements
not only by the hydrogens but by alkali metals, etc. Hydrogen is, it may
here be remarked, an element of unique character; not only can it be
replaced by the elements of the widely different classes represented by
chlorine and sodium, but it is the terminal of the series of paraffins,
C_{n}H_{2n}; C_{3}H_{6}, C_{2}H_{4}, H_{2}. The third proposition which
must be taken for granted is, that the groups of elements, C_{2}H_{5},
CH_{3}, behave as elements, and that these radicals, ethyl, methyl,
etc., do not suffer decomposition in many chemical reactions.
Now as to valency or atomicity, accepting the received atomic weights of
the elements, it is certain that there are at least four distinct types
of hydrogen compounds represented by ClH, OH_{2}, NH_{3}, CH_{4}. The
recognition of these types, and their relations to each other as
types, was one of the most important and best assured advances made in
theoretical chemistry. When we compare the formula of water with that of
hydrochloric acid, we find that there is twice as much hydrogen combined
with one atom of oxygen as there is combined with one atom of chlorine;
and in a great many other instances, we find that we can replace two
atoms of chlorine by one atom of oxygen, so that we get an idea of the
exchangeable value of these elements, and we say that one atom of oxygen
is worth two of chlorine, or is bivalent; similarly, nitrogen is said to
be trivalent. The meaning attached to the word "valency," is simply one
of interchangeability, just as we say a penny is worth two halfpennies
or four farthings. The question next arises, is the valency of an
element fixed or variable? If the word be defined as above, it is
absolutely certain that the valency varies. Thus, tin may be trivalent,
SnCl_{2}, or tetravalent, SnCl_{4}. Accordingly elements have been
classed as monads, dyads, triads, etc. The lecturer objected most
strongly to the word "atomicity;" he could not conceive of one atom
being more atomic than another; he could understand the atomicity of
a molecule or the equivalency of an atom, but not the atomicity of an
atom; the expression seemed to him complete nonsense. He next considered
the possibility of assigning a fixed limit to this valency or adicity of
an atom, and concluded that the adicity was not absolutely fixed, but
was fixed in relation to certain elements, e.g., C never combines with
more than four atoms of H; O never more than two atoms of H, etc. The
adicity of an element when combined with two or more elements is usually
higher than when combined with only one, e.g., NH_{3}, NH_{4}Cl. The
term "capacity of saturation," may be used as a synonym for adicity, if
care be taken to distinguish it from other kinds of saturation, such as
an acid with an alkali, etc. Adicity is, however, quite distinct from
combining force; the latter is indicated by the amount of heat evolved
in the combination.
The lecturer then proceeded to criticise a statement commonly found
in text books, that chemical combination suppresses altogether the
properties of the combining bodies. The reverse of this statement is
probably true. To take the case commonly given of the combination of
copper and sulphur when heated; this is good as far as it goes, but
there are numerous instances, as ClI, SSe, etc., where the original
properties and characters of the combining elements do not completely
disappear. The real statement is that the original properties of the
elements disappear more or less, and least when the combination is weak
and attended with the evolution of a slight amount of heat, and in every
case some properties are left which can be recognized. So with reference
to the question of atomic and molecular combination, as atomic
combination does not necessarily produce change, it does not differ in
this respect from what is usually called molecular combination.
The lecturer then referred to an important difference in the adicity of
chlorine and oxygen. Chlorine can combine with methyl or ethyl singly.
Oxygen can combine with both and hold them together in one molecule. The
recognition of this fundamental difference between chlorine and oxygen,
this formation of double oxides as opposed to single chlorides, marks an
epoch in scientific chemistry.
The lecturer then considered the subject of chemical formulæ; it is the
bounden duty of every formula to express clearly the number of atoms of
each kind of elementary matter which enters into the constitution of the
molecule of the substance. A formula may do much more than this. If we
attempt to express too much by a complex formula we may veil the number
of atoms contained in it. This difficulty may be avoided by using two
formulæ, a synoptic formula giving the number of atoms present, and a
complex formula perhaps covering half a page, giving the constitution
of the molecule. But between the purely synoptic formula and the very
elaborate formula there are others--contracted formulæ--which labor
under the disadvantage, as a rule, of being one-sided, and so create a
false impression as to the nature of the substance. Thus, for instance,
to take the formula of sulphuric acid, H_{2}SO_{4}. This suggests that
all the oxygen is united to the S; (HO)_{2}SO_{2} suggests that two
atoms of hydroxyl exist in the molecule; then, again, we might write the
formula HSO_{2}OH, or H_{2}OSO_{3}. All of these are justifiable, and
each might be useful to explain certain reactions of sulphuric acid, but
to use one only creates a false impression. The only plan is to use them
variously and capriciously, according to the reaction to be explained.
Again, ethyl acetate may be written--
H_{3}C\
H_{2}C/
\
O
/
OC\
H_{3}C/