Only weeks before a chemistry experiment sent a plume of fire across a Manhattan high school science lab, engulfing two students and leaving one with life-threatening burns, a federal safety agency issued a video warning of the dangers of the very same experiment, a common one across the country.

The agency, the United States Chemical Safety Board, distributed the video warning to its 60,000 subscribers, a spokeswoman, Hillary Cohen, said Friday, but it had no sure way to reach individual teachers at schools like Beacon High School on the Upper West Side. There on Thursday, Anna Poole, a young science teacher known for safety consciousness, used methanol as an accelerant to burn dishes of different minerals in the chemistry demonstration known as the Rainbow.

With about 30 students watching from their desks, a snakelike flame tore through the air, missing the students closest to the teacher’s desk, but enveloping Alonzo Yanes, 16, searing and melting the skin on his face and body, according to witnesses. He was in critical condition on Friday in the burn unit of NewYork-Presbyterian Hospital/Weill Cornell Medical Center, Myrna Manners, a hospital spokeswoman, said.

Another student, Julia Saltonstall, 16, saw her thin T-shirt burned off her torso in an instant as some of her long dark hair went up in smoke, her father said. Though she was no farther from the demonstration than Alonzo, she escaped with only first-degree burns.