884 
leading directly to a symbolism and indirectly to a 
mechanical interpretation. Each atom is conceived 
as having a definite number of places for the attachment 
of other atoms, and as the number is in each case small 
we can conveniently represent it in a graphic symbol. 
It is not without interest to look back to the origin 
of graphic or constitutional formule and see the 
beginnings of our conventional system. Couper and 
Kekulé, the originators of the idea, suggested systems 
widely differing from each other. Couper (1858) 
symbolised acetic acid as follows, 
C=; Seas 
in appearance a near approach to present-day usage if 
we allow for the fact that he assumes O=8 and C=12. 
The manner of linking of various atoms is indicated, 
but their valency is not clearly symbolised. Kekulé’s 
graphic formula (1859) for the same substance is shown 
below. 
Acetic Acid. 








I 
fy 
HRs 

loll 
A) 

(Kekulé.) 
The valency is satisfactorily represented, but the link- 
age of the atoms is confused. Only atoms touching 
in a vertical line are supposed to be directly linked. 
The system is cumbrous, and Kekulé himself used it 
but sparingly. For branched chains it becomes im- 
practicable. 
Loschmidt (1861) devised a clear logical system 
which, although he formulated by its means hundreds 
of compounds, some of a very complex nature, found 
no favour amongst chemists. His symbol for acetic 
acid is given in the figure. Here linkage and valency 
are adequately represented, but the atomic symbols are 
arbitrary, and the system, like Kekulé’s, is cumbrous 
to use. 
Simultaneously with Loschmidt, Crum Brown (186r), 
although he was unaware of Couper’s work, used a 
symbolism resembling his, and practically identical 
with that in current use. He writes glycol as follows : 
(Loschmidt.) 
@ 
@® cas 2} Kanon: © Asacnsner ©-® panes, ® 
6 
Here valency and linkage are clear, and the atomic 
symbols are no longer arbitrary. Wurtz in 1864 gave 
the following formula (I.) for the same substance : 
H 
9 H 1 
(.) €H, (II.) 
i= oC One it 
(is | 
$ iH 
H 
The line of development of modern graphic formule, | conception of number of equivalents. In 
NO. 2800, voL. 111] 
NATURE 

[JUNE 30, 1923 
then, is through Couper, Crum Brown, and Wurtz, and 
not through Kekulé or Loschmidt. The reason for 
the preference shown by chemists is that the symbolism 
adopted is more obvious, simpler to use, simpler to 
reproduce, and easily adaptable to all organic com- 
pounds. 
There is a dual character in our graphic formule 
which it is important that we should realise. Let us 
begin with the graphic symbols of the elements, thus : 
H-, ety. et where each short line represents a 
unit of valency. To the graphic symbols of molecules 
is but a step; the atoms are represented as united 
together, the valencies indicating the manner of 
attachment (II.). The directness or indirectness of 
union of the atoms is here given ; no hydrogen atom is 
in this formula directly attached to another ; they are 
only indirectly united through carbon or oxygen. For 
brevity, we join up the lines representing the valencies 
of the various atoms and obtain the customary formula 
(IIL.). A great change in significance has, however, at 
this stage taken place: the valencies have become — 
“ bonds ”—the idea of force has entered. What that 
force is remains indeterminate ; it is merely something 
binding atoms together, and the interpretation of the 
symbols is not quantitative in this respect. The 
“bonds ” do not represent equal forces ; it is patent 
that the bond between hydrogen and carbon in the 
formula for methyl alcohol represents a different force 
from that between hydrogen and oxygen. The current 
symbolism may then be interpreted in two senses ; 
the lines joining the atomic symbols may be taken 
to represent on one hand merely the union of the~ 
atoms to the symbols of which they are attached, or 
they may represent forces existing between these 
atoms. Confusion of these two senses sometimes leads 
to erroneous reasoning. de 
This dual character of graphic formule is noticeable 
from their earliest employment, and I need make no 
apology in illustrating it from the work of Crum Brown, 
whose recent death deprived our Society of its senior — 
past-president. In the formula for glycol quoted above ~ 
he states (1861) that the dotted lines therein employed 
represent polar forces. In 1864 he represents ethane 
thus (IV.), 
(IV.) @-- (V.) 
--©--® 
--©--@ I 
| ©—O—O—® 
o) 
each valence being written independently. He says : 
““T may here shortly explain the graphic notation 
which I employ to express constitutional formule. — 
... An atom is represented by its usual 
symbol surrounded by a circle with as many 
lines proceeding from it as the atom con- 
tains equivalents. . . . When equivalents 
mutually saturate one another the two lines — 
representing the equivalents are made con- 
tinuations of one another.” Here he has 
discarded the idea of polar force in his— 
original formulation in favour of the simpler 
1866 © 


