74 NATURE 
elements devoid of chemical properties, but by a 
second series, known only in minute quantities, and 
displaying those extraordinary properties of radio- 
activity which have revolutionised our ideas in more 
than one direction. 
It is not necessary for me to chronicle even the 
more striking achievements in chemistry since 1886; 
a few examples will show how great the progress 
has been. It is on record that Arrhenius was present 
at that meeting, but his advocacy of that theory of 
solutions with which his name will always be asso- 
ciated came a little later; phenylhydrazine, which was 
to play so important a part in Emil Fischer’s investi- 
gation of the sugars, had been discovered by him 
only two years previously; the Grignard reagent, 
which in other directions has played a no less impor- 
tant part in synthetical organic chemistry, did not 
become available until some fourteen years later. 
Theories then emerging, such as that of geometrical 
isomerism, have either been discarded or modified by 
the discovery of new facts, and who shall say that 
the ionic theory of dissociation stands where it did, 
now that ions in solution have incurred the suspicion 
of associating with the solvent, and to that extent 
have come into line with molecules, for the orthodox 
behaviour of which Prof. Armstrong himself would 
no doubt be prepared to vouch. 
Residual Valency. 
Among the many doctrines which have suffered 
under the stress of long-sustained investigation, that 
of valency is a prominent example. Valency is that 
property by which an atom attracts to itself other 
atoms or radicals, and its numerical value is deduced 
from the structural formule of compounds in which 
that atom occurs. Claus seems to have been the first 
to recognise that this attraction between two atoms 
is not a constant, but depends on the nature of the 
other atoms or radicals in the molecule,+ and it is 
of interest to note in connection with what follows 
that he used methane and its chloro-derivatives to 
illustrate his point of view. Valency may vary, there- 
fore, from compound to compound; it is known to 
alter under the influence of change in temperature, 
as, for example, when carbon dioxide or phosphorus 
pentachloride undergoes thermal dissociation. But 
Claus’s view did not meet with ready acceptance; 
hence at the Birmingham meeting few chemists, if 
any, would have questioned the quadrivalency of 
carbon, despite the difficulty caused by the existence 
of carbon monoxide. Now, carbon is believed to be 
bivalent in the carbamines, fulminic acid and other 
compounds, as well as in carbon monoxide, and its 
tervalency is coming to be accepted in the light of 
the latest investigations on triphenylmethyl and its 
congeners. What is true of carbon is equally true of 
all other elements, except argon and its companions. 
Hence the doctrine of constant valency for which 
Kekulé contended, or that of variable valency in 
which the uncombined units varied by even numbers 
has necessarily given place to one of less rigid type, 
although the final form has yet to be determined. 
For the purpose of this address it will be sufficient 
to refer only to one of these later theories: that in 
which Werner, as the outcome of his exhaustive study 
of inorganic molecular compounds, and especially of 
the ammines, supposes that an atom may have both 
principal and auxiliary or residual valency. There 
4 A. Claus, Ber, 1881, xiv, 435. It may be noted that Claus concludes 
his paper with the statement, “. . . die Annahme von Valenzen, als in 
den mehrwerthigen Atomen priiexistirender ihrer Wirkungsgrisse nach 
bestimmter Anzichungseinheiten eine ebenso unbegriindete, wie unnatiirliche 
Hypothese ist.” 
5 A, Werner, “‘ Neuere Anschauungen auf dem Gebiete der anorganischen 
Chemie" (Friedr. Vieweg u. Sohn, Braunschweig, 1908): English edition. 
““ New Ideas on Inorganic.Chemistry.” E. P. Hedley. (Longmans, rorr.) 
NO. 2290, VOL. 92] 
[SEPTEMBER 18, 1913 
| are difficulties in its application to certain problems 
| of organic chemistry—for example, the structure of 
| the benzene molecule—but the conspicuous success 
which has attended Werner’s ifivestigation of the 
complicated isomerism of the cobalt and chromium 
ammines is evidence of its value as a guide in stimu- 
lating research in the most unpromising directions.® 
Werner’s view that valency is an attractive force 
acting from the centre of the atom, being of equal 
value at all points on the surface and independent of 
units of affinity, has the merit of meeting the objec- 
tion long urged to the idea that affinity has fixed 
direction in space, but otherwise leaves untouched 
Van’t Hoff’s brilliant conception of asymmetry which 
plays so great a part in the chemistry of to-day. 
What light does this conception of residual valency, 
dating back to 1885, if not earlier,? and now em- 
bodied in many theories besides Werner’s, throw on 
some of the problems with which the organic chemist 
is faced? Much every way. The question of the dis- 
tribution of valency in the molecules of carbon com- 
pounds is discussed probably more than any other; 
it arises in connection with the structure of un. 
saturated compounds, the properties of fluorescence 
or colour which many of them exhibit, and the rela- 
tion between chemical constitution and physical pro- 
perties, to the elucidation of which an increasing 
amount of research is being directed. The double 
linkings in our formule no longer represent two 
units of valency in terms of hydrogen, nor are they 
now used to indicate polarity of the central atom or 
distribution of the valency in space; Werner’s con- 
ception of valency accounts, as the phrase goes, for 
the concentration of re-activity at that part of the 
molecule where unsaturation exists, and it is of ser- 
vice when different degrees of unsaturatedness are 
displayed by compounds which, on the older view, 
would be expected to show similarity in chemical 
behaviour. With your permission I propose briefly 
to review our knowledge of that type of chemical 
change known as substitution from the point of view 
of residual valency. 
Substitution in the Paraffin Series. 
So far back as 1839 the fact was discovered that 
replacement of hydrogen by chlorine in the acetic 
acid molecule does not lead to any essential modifica- 
tion in the properties of the acid. It is not a little 
remarkable, therefore, that although much of the pro- 
gress in organic chemistry has been achieved by sub- 
stitutions of the most diverse types, we are still 
unable to say that agreement has been reached with 
regard to the nature of the processes by which this 
replacement of one radical by another in a molecule 
is brought about. Never has attention been concen- 
trated more closely than now on the study of what, 
for want of a better phrase, is termed the “mechan- 
ism of chemical reactions"—the processes which are 
covered and hidden by the sign of equality used, 
inaptly, in chemical equations—but the integratin 
mind, to the need for which Professor Franklan 
alluded on a recent occasion,® has not yet been evolved 
to reconcile the uncertain or contradictory answers 
vouchsafed to much patient experimenting. Organic 
6 A. Werner, Fer, 1011, xliv, 2445, 3231. 
7S._U. Pickering, Proc. Chem. Soc., 1885, i, 122; H. E. Armstrong, 
Proc. Roy. Soc., 1886, x], 285. 
As an example of the unsatisfactory character of the doubly-linked 
formula to which the older meaning was attached, the following may be 
quoted : unsym.-Diphenyldichloroethylene, like ethylene, combines mole- 
cularly with bromine, but tetraphenylethylene does not : 
ros {Colas C(CgH5)2 
CH2 C Cle C(CgH5)o 
yet asimilar structure has been assigned to each (Biltz, Annalen, 1897 
ccxcvi, 219). 
* P. F. Frankland, Proc. Chem. Soe., 1913, XXiX, TOL. 
