Number 172 (Story #2), April 7, 1994 by Phillip F. Schewe and Ben Stein POROUS SILICON , unlike bulk silicon, can emit light, a fact that may lead to new opto- electronic devices. Studies over the past few years have determined that the luminescence arises not from any surface residue left behind by the silicon electrochemical etching process but rather from some quantum effect relating to the small size of the silicon structures responsible for light emission. A new experiment at AT&T Bell Labs (Louis Brus, 908-582-3119) shows that these structures are not wire-like, as was previously thought, but more particle-like with a typical size of only 2 or 3 nm. (Upcoming article: S. Schuppler et al., Physical Review Letters, 18 April.)