298 SECTIONAL TRANSACTIONS.—B. 
of the reaction HO + HD = HOD + Hyg is 3°8 at 20° and 1-8 at 100° C. 
It is pointed out that this equilibrium reaction may play an important réle 
in the electrolytic separation of the hydrogen isotopes. ‘The same applies 
also to the separation observed in dissolving metals in water or in acids in 
presence of heavy water. In the case of the photochemical hydrogen- 
chlorine reaction it is shown that the heavy hydrogen reacts slower than the 
light, since its activation energy for the first step of this chain reaction is 
larger owing to its smaller zero-point energy. In collaboration with Prof. 
Rideal we have found that in the catalytic interaction of heavy hydrogen and 
ethylene two reactions take place simultaneously but independently: the 
addition of hydrogen and the exchange of hydrogen. With Mr. Yudkin 
we have investigated the enzymatic decomposition of sodium formate in 
presence of heavy water by B. coli. The hydrogen evolved is in equilibrium 
with the water, and also the interaction H,O + HD = HOD + H, is 
readily catalysed by the bacteria. 
Mr. C. StracHan.—Adsorption of gaseous tsotopes. 
The possible energy states of an atom or molecule adsorbed on the surface 
of a solid have been considered by the methods of wave mechanics. ‘The 
solid is treated as in the theory of specific heats developed by Born, Debye 
and others. The adatom is supposed held by forces giving a potential 
energy which varies somewhat as shown with distance from the surface. 
Potential 
Energy 
Distance from surface ————> 
Under the influence of the heat motion of the solid the adatom can take 
up states of different energy in this potential energy trough and can perhaps 
evaporate from the surface. 
A knowledge of heat of adsorption, difference of zero-point energy for 
isotopes, and results from adsorption isothermals can give quantitative 
information about the parameters involved in the description of the above 
potential energy curve. The analysis then allows the evaluation of: 
(1) Average time intervals between transitions of adatoms from ‘ bound ’ 
states to states when surface migration is possible, (2) probability of evapora- 
tion, and (3) length of life of adatom in ‘ migratory’ state, together with 
their dependence on temperature and the differences of behaviour of isotopes. 
In particular, results are obtained for hydrogen (H and D) adsorbed on 
