With the discovery of the pion in 1947, the search for new elementary particles became a popular research topic. By operating fast ionization chambers within a cloud chamber, Herbert showed that the bursts of ionization they recorded were primarily produced by relatively low energy cosmic rays, whose nuclear interactions typically involve the ejection of several heavily ionising nuclear fragments. On the basis of this effect, he and Rossi demonstrated that the behaviour of these interactions is similar to that of penetrating showers.