- > 
June 30, 1923] 
NATURE 
883 

‘ Chemical Symbols and Formule.* 
By Sir James WALKER, F.R.S. 
YMBOLS are both an aid and an obstacle to 
thought. Their brevity and simplicity may help 
us, working according to a fixed system, to perform 
mental operations which without their aid might be 
practically impossible. Their generality too may, as 
in algebra, enable us to solve thousands of problems in 
one. On the other hand, we sometimes find in science 
a system of symbols which, at first of great value, may 
in virtue of its very success so warp our thought or 
limit our mental outlook as to constitute a real hindrance 
to scientific progress. There is always -the danger, 
arising from our familiar and constant use of the 
symbol, either of forgetting what it properly symbolises, 
or of confusing the symbol with the thing symbolised. 
The function of the symbol is a practical one ; in 
Mach’s phrase, it is to effect economy of thought, and 
it is precisely because mankind at large is so economical 
of thought that the dangers of symbolism originate. 
The danger, however, must be faced by the student of 
chemical science, for without symbols systematic 
advance is impossible: the symbols are based on a 
theory and permit the representation of that theory 
ih detail. 
If we examine the practical requirement of a satis- 
factory system of symbols, we shall find that the system 
must be simple in itself and simple to operate. Con- 
sider the Roman schoolboy confronted with the problem 
of multiplying MCMXXIII by CXLIV. The system 
of notation is not too complicated, but to operate with 
_it is practically impossible. To perform his task he 
must abandon the symbolism and have recourse to 
concrete objects—the fingers or an abacus. The Arabic 
notation, on the other hand, with its consistent valua- 
tion by position and the introduction of a symbol for 
zero, enables us, once we have passed the barriers of 
the addition and multiplication tables, to perform 
arithmetical calculations of all kinds with ease and 
speed. It is simple in itself and simple to operate. 
The same requirements are essential to a system of 
chemical symbols. The first symbols, those for the 
metals known to the ancients, indicated nothing but 
their supposed association with the planets and the 
gods ruling them. Thus the solar disk stood for 
gold, the lunar crescent for silver, the mirror of 
Venus for the Cyprian metal copper, and so on. 
Towards the end of the eighteenth century we see 
the beginnings of our present system of elementary 
symbols. Hassenfratz and Adet (1787) used for the 
non-metals straight and curved lines which could be 
combined together (much as in phonetic shorthand) to 
represent the qualitative composition of compounds, 
The symbol for a metal was a circle, and to distinguish 
one metal from another the initial of its Latin name 
was written within the circle—thus (Sb) was the symbol 
for antimony. 
Dalton used for metals and non-metals alike only 
circular symbols, doubtless to represent spherical atoms, 
and in his hands the symbols assumed a quantitative 
significance based upon his atomic theory. For the 
simple non-metals these symbols were arbitrarily 
4 Presidential address delivered at the annual general meeting of the 
Chemical Society on March 22. 
No. 2800, VOL. 111] 
chosen, © representing an atom of oxygen, © an atom 
of hydrogen, ( an atom of nitrogen, and so on. For 
the metals he adopted the same device as Hassenfratz 
and Adet, using, however, the English instead of the 
Latin names, so that for example (x) represented an 
atom of lead. Compounds could be represented by 
the juxtaposition of the elementary symbols, which 
now gave, not only the qualitative, but also the 
quantitative composition of the compound. Thus, for 
Dalton, water was represented by the symbol OO, 
denoting the combination of 7 parts of oxygen with 
1 of hydrogen. 
_ Berzelius (1815) took the final step by using Latin 
initials for all the elements, dropping the circles which 
had surrounded them, and employing affixed numerals 
to indicate the number of times the symbol had to be 
fepeated. It is true that Berzelius spoiled the uni- 
formity of his system by using a special dot symbol 
for oxygen and writing such formule as S for sulphur 
trioxide. These dotted symbols, however, found little 
favour except amongst mineralogists, and gradually 
passed out of use. The disuse of the circles is not 
without significance—the symbol to Berzelius repre- 
sented a combining weight rather than a concrete 
atom, and the dual quantitative use persists in the 
interpretation of symbols to-day. The symbol C 
Stands for one atom of carbon or “ twelve parts by 
weight ” of carbon. So we may say that more than a 
hundred years ago a system of formulation had been 
reached which, with minor alterations, is in use at the 
present time for the representation of elements and 
the composition of compounds, and is never likely to 
be superseded. It is uniform, plain, and simple in 
itself, and simple to use in the equations representing 
chemical change. 
* The purely compositional formule, however, fall far 
short of expressing what calls for expression in various 
classes of chemical compounds: action and structure 
have to be considered as well as composition. The 
dualistic formule of Berzelius illustrate early attempts 
in this direction. The formula of sodium sulphate is 
hot written empirically as Na,SO,, but dualistically as 
Na,O,SO,. This formula indicates inter alia that the 
sodium and the sulphur belong to two essentially 
different parts of the compound. The modern electro- 
chemical dualism writes Na*,SO,", again indicating the 
same division of a positive from a negative portion. 
In organic chemistry the representation of structure 
by means of formule achieved success by the clear 
recognition of valency—in particular, the quadrivalence 
of the carbon atom. At this point of development the 
notion of the atom as structural unit becomes -in- 
dispensable. 
The valency of an element on its experimental side 
is in essence a numerical conception. We divide a 
weight by a weight, namely, the atomic weight by the 
equivalent weight, and obtain in consequence a mere 
number. When we pass from element to atom, how- 
ever, the conception undergoes a transformation, and 
receives a concrete meaning. The valency of an atom 
may be interpreted as its capacity for combining with 
other atoms, again a numerical conception, but one 
