fissile

When scientists first used fissile back in the 1600s, the notion of splitting the nucleus of an atom would have seemed far-fetched indeed. In those days, people thought that atoms were the smallest particles of matter that existed and therefore could not be split. Fissile (which can be traced back to Latin findere, meaning "to split" or "to cleave") was used in reference to things like rocks. When we hear about fissile materials today, the reference is usually to nuclear fission: the splitting of an atomic nucleus that releases a huge amount of energy. But there is still a place in our language for the original sense of fissile (and for the noun fissility, meaning "the quality of being fissile"). A geologist, for example, might refer to slate as being fissile.