The US Chemical Safety Board recently issued a video warning of the dangers of the “Rainbow” experiment, a common one across the country.

Despite this waning, a Manhattan high school ran that experiment — resulting in an accident that sent a plume of fire across the science lab, engulfing two students and leaving one with life-threatening burns.

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.

“She came so close to being so much more badly hurt,” said her father, David Saltonstall, who had just started his first day as policy director for Scott M. Stringer, the New York City comptroller, when he got a 9:30 a.m. call from the school nurse that his daughter was in an ambulance heading for the hospital. His daughter called from the ambulance moments later, her voice quavering as she described the terrible accident.

“All of us feel just so terrible for Alonzo and the struggle he has ahead,” Mr. Saltonstall said, describing the boy, a classmate of Julia’s since middle school, as “funny and bright.” His parents, who live in the Bronx and were holding vigil at the hospital, declined to speak about their son’s condition.

The safety board’s video, and an accompanying message, did not say the Rainbow demonstration should be banned, but warned that accidents have repeatedly occurred because of the volatile material involved.

“What we need to look at is why is this accident keeps happening across the country,” said Mary Beth Mulcahy, a former high school science teacher who is now an investigator with the safety board, which has documented at least seven similar accidents, including a 2006 case featured in the video that left a 15-year-old girl in Ohio, Calais Weber, with severe burns over more than 48 percent of her body. “What do we need to do to stop the cycle?”

As a 23-year-old teacher, Dr. Mulcahy added, she herself did the rainbow demonstration, unaware of the potential dangers. The visually exciting demonstration shows how different substances produce flames of different colors because of their varying properties. But she added, “I can’t imagine a teacher would do this demonstration if they knew the potential risk they were putting students in.” risk they were putting students in.”