414 TRANSACTIONS OF SECTION B. 
Dealing with these alternatives in the order given, Arrhenius adopts a view 
of the process of substitution which, including as it does his explanation of 
optical inversion and racemisation, should perhaps be given in his own words :— 
‘Every valency linking can be broken; this is true in all cases, since it is a 
necessary condition for every chemical reaction, An atom or an atomic complex 
is thereby removed from the molecule, and its place taken by another atom or 
atomic complex. One must therefore assume, as was first’ pointed out by 
Williamson, that the atoms or complexes separate themselves from the molecule 
from time to time, even when they do not react with other molecules. Con- 
sider now a molecule in which four different atoms, A, B, C, and D, are bound 
to one carbon atom. The atoms A and B, which may possess equal charges, 
e.g., positive, are therefore separated at times from the molecule, and it may 
happen that they are both separated at one and the same time. It is therefore 
possible for them to change places on combining with the carbon atom again. 
This is synonymous with a transformation of the original molecule into its 
optical isomer.’ *? 
Nef, making use of ‘the conception of dissociation in its broadest sense,’ is 
of opinion that the decomposition of ethane into hydrogen and ethylene at 800° 
“proves that an extremely small per cent. of [its] molecules must exist at ordinary 
temperature in an active or dissociated condition, 
CH;:CH; —> CH,°CH,+ H—’ 
consequently, when ‘chlorine reacts with ethane to give the monochloro- substitu- 
tion product, we have this reagent in the active molecular condition simply 
uniting by addition with the dissociated ethane particles, 
js filet a lle t | [> HCl + 0,H,C1] 
oH; 
Finally, he draws the conclusion that ‘ excluding reactions called ionic, a chemical 
reaction between two substances always first takes place by their union to form 
an additive compound.’ 
Michael,?* in many published papers, has emphasised the view that in the 
substitution of halogen for hydrogen in a saturated hydrocarbon or saturated 
acid the principal factors to be taken into account are the mutual chemical 
attraction of the two elements, on the one hand and that of the halogen and 
carbon, on the other. By applying his ‘positive-negative’ hypothesis to the 
directing influence of ‘relatively-positive’ methyl, and ‘relatively-negative’ 
carboxyl, he draws conclusions about the degree of firmness or looseness with 
which particular hydrogen atoms are bound to carbon in the molecule, and is 
thereby able to forecast with some success the position or positions in which 
replacement of hydrogen by halogen will occur. Flirscheim, in the discussion 
of the relation between the strength of acids and bases, and the quantitative 
distribution of affinity in the molecule, also makes use of the idea that the 
relative degree of firmness or looseness with which a hydrogen atom is held 
depends on the nature of the other atoms or radicals associated with the same 
carbon atom.** The hydrogen atoms therefore are not to be regarded as retained 
in the molecule with the same degree of firmness; in other words, valency is not 
a constant to be measured in units. 
It will be gathered therefore that Arrhenius and Nef, from different stand- 
points, support the idea that separation of hydrogen from the hydrocarbon 
precedes entry of the substituent into the molecule; Michael and Fliirscheim are 
concerned chiefly with the distribution of valency in the molecule, which deter- 
mines whether a particular hydrogen atom shall be displaced by hydrogen or 
218. Arrhenius, Theories of Chemistry, edited by T. Slater Price (Longmans, 
1907), p. 76. 
22 J. U. Nef, ‘The Fundamental Conceptions underlying the Chemistry of 
the Carbon Atom,’ J. Amer. Chem. Soc., 1904, 26, 1566. 
23 A. Michael, Ber., 1901, 84, 4028, covering reference to earlier papers. 
24 B. Fliirscheim, 7’rans. Chem. Soc., 1909, 95, 721. 
