On television, detectives mist a spotless room, kill the lights, and a hidden crime scene glows blue. The real chemical behind that scene is luminol, a white to pale yellow powder that does not dissolve in water and does nothing on its own. Mixed with hydrogen peroxide in a basic solution and given a catalyst, it oxidizes into an unstable molecule that drops from an excited state to its ground state and throws off light at about 425 nanometers. At a crime scene the catalyst is the iron in hemoglobin, and if the pH falls under 8.2 the reaction produces no light at all.

This episode explains that reaction and the slow, accidental path from lab curiosity to forensic tool. A German chemist first made the compound in 1902 with no thought of police work, and the name luminol did not appear until 1934. Others noticed in 1928 that blood strengthened the glow, tied the effect to hematin in 1936, and tried it on crime scenes in 1937. The episode also covers the limits: the glow lasts about 30 seconds, and luminol is only a presumptive test that tells investigators where to look, not what they have found.

• Old, dried blood glows more strongly than fresh blood because burst red cells leave their iron exposed.
• In 1939 San Francisco pathologists Frederick Proescher and A. M. Moodie showed that large areas could be sprayed quickly and that a stain could be made to glow again and again.
• Copper, household bleach, fecal matter, and cigarette smoke residue all trigger the same blue light, and so does horseradish, which is packed with peroxidase enzymes.
• Because the glow fades so fast, investigators document it with long exposure photography in a darkened room.
• Luminol can degrade genetic material, but modern amplification methods such as PCR usually still recover a DNA profile from treated samples.