Becquerel rays

The Becquerel rays are invisible, and only known by their effects, which are of various kinds: thus, like the R�ntgen rays, they blacken a photographic plate, even after passing through glass or other intervening substances; they cause a number of different substances to give out a fluorescent light, and they render air a conductor of electricity.

And what seemed the more wonderful was the fact that these "Becquerel rays," as they were now called, emanated spontaneously from the pitch-blende.