Nikolai N. Semenov - Nobel Lecture - Nobelprize.org

492 1956 N.N.SEMENOV
ergetic reaction occurs if one of these parameters varies within extremely
close limits which, on some occasions, are virtually unmeasurable. Similar
limit phenomena were observed by Sir Cyril Hinshelwood and collaborators
19 in the case of pressures which exceed a certain "second upper limit"
(e.g. in the case of the reaction of water formation from oxygen and hydrogen).
The second upper limit was explained on a basis of the ideas put
forward by Sir Cyril Hinshelwood regarding chain termination in the volume
in the case of threefold collisions and the concept of chain branching.
This meant that both our investigations and those conducted by Sir Cyril
Hinshelwood 19 and his collaborators led to the establishment of a new idea
of the reaction limit, as the result of each branched process which is accompanied
by chain termination. These researches were conducted at the
end of the twenties and start of the thirties.
When, at the end of the thirties, the physicists discovered branched nuclear
reactions, similar limit phenomena were observed as with the chemical
reactions, in spite of the difference which obtained in principle, because
limit phenomena of this type characterize any branched chain reaction.
With nuclear reactions, it is possible to observe with particular clarity
the transition from the perfectly inert condition to an accelerated reaction
or even explosion, following a change in such parameters as size and density,
etc. (for example when control rods are introduced into or moved out
of nuclear reactors).
From the field of the slow chemical reactions, we ought first to quote
chain reactions with degenerate branching. These reactions are characterized
in that their primary product is a molecular intermediate which enters
relatively slowly into the reaction leading to the formation of free radicals.
The existence of such reactions was proved even in the thirties, when it was
established that the reactions of hydrocarbon oxidation are degenerately
branched. The theory of "degenerate explosions", which we developed in
1931-1934 20 is today generally accepted.
As is shown by the work of A. Ya. Apin 21 , not only the oxidation processes
but also the decomposition of nitrogen chloride has the character of a
branched chain reaction with an upper ignition limit.
Most other chemical gas reactions constitute unbranched chain reactions.
With such reactions and also with the branched reactions, chain propagation
takes place rapidly due to the formation of free radicals. Since however
there are no branchings, there is no spontaneous development. Each chain
is produced only by reason of the action of a spontaneously forming free