Page "Chain reaction" Paragraph 3

In 1913 the German chemist Max Bodenstein first put forth the idea of chemical chain reactions.

If two molecules react, not only molecules of the final reaction products are formed, but also some unstable molecules, having the property of being able to further react with the parent molecules with a far larger probability than the initial reactants.

In the new reaction, further unstable molecules are formed besides the stable products, and so on.