The chlorine atoms are now in an excited, energy-rich state. 

    These atoms, in their turn, bear down upon the hydrogen molecules and tear them apart into atoms too. One of the latter combines with a chlorine atom and the other remains free. But it is excited. It craves to give away part of its energy. To whom? Why, to a chlorine molecule. And when it collides with one, that is the end of the phlegmatic chlorine molecule.

    And now again there is an active chlorine ion at large, but it does not take long for this atom to find an outlet for its energy.

    Thus we get a long consecutive chain of reactions.

    As soon as the reaction starts, more and more molecules are activated by the energy liberated as a result of the reaction. The rate of the reaction increases like an avalanche of snow rolling down a mountain. 

    When the avalanche reaches the valley it dies down. The chain reaction dies out when all the molecules have been caught up by it, when all the hydrogen and chlorine molecules have reacted.

    Chemists know multitudes of chain reactions. Our prominent scientist Nikolai Semyonov has studied how these reactions occur in great detail. 

    Chain reactions are known to physicists too. The fission of uranium atoms is an example of a physical chain reaction.

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