Four children who were using combustible materials in a chemistry experiment at a Northwest Washington elementary school were injured yesterday morning, two of them critically, when the materials exploded.

The blast occurred in a science laboratory at Murch Elementary School, on 36th and between Ellicott and Davenport streets NW, where 14 students enrolled in a summer enrichment program for gifted youngsters were making fireworks commonly known as "sparklers," D.C. police officials said.

D.C. school spokeswoman Janis Cromer later called the experiment "inappropriate" and said the materials used, which are similar to those used in gunpowder, were "not supposed to be in the schools whatsoever."

The program at Murch, called "Summer Discovery," is paid for by parents at $165 per pupil and is supervised and taught by educators hired by the program. School system personnel and funds are not involved. Cromer said that when school administrators reviewed plans for the program several weeks ago, there was "no mention of the sparklers experiment."

Two 9-year-old boys injured by the explosion were taken by helicopter to Children's Hospital, where they were admitted to the intensive care unit in critical condition.

One boy suffered second- and third-degree burns over 20 percent of his body, with his face, arms and hands badly burned, according to a hospital spokeswoman. The other was said to have suffered burns over a slighty smaller portion of his body.

The two 9-year-olds were identified as Derreck Howell, a student at Eaton Elementary School, and Stewart Ugelow, who attends Murch, school officials confirmed. It was unclear which boy had the more serious injuries.

In addition, two 8-year-olds, a girl and a boy, were taken by ambulance to Children's, where they were listed in good condition last night. The girl, identified as Puja Malholtra, was slightly burned, while the boy, identified as Jonathan Foer, suffered shock as a result of the violent blast but was not burned. The two, both of whom attend Murch, were being held for observation last night, officials said.

The students had been instructed to mix the combustible substances -- potassium perchlorate, sulfur, charcoal, iron powder and aluminum powder -- in large bowls and grind them as fine as sugar, according to Charles Butta, director of the two-week program.

Later in the week, they were to use an adhesive to apply the powdery mixture to wires from coat hangers to make sparklers similar to those used during Independence Day celebrations.

Moments before the explosion, one of the students apparently disobeyed instructions and started jamming a wire into his bowl of substances. The impact of the wire apparently created a spark that ignited the powders, said Butta, who described the accident as "horrible" and "traumatic."

Michael Nickens, 10, who was participating in the experiment but was not injured, described it as a fun exercise that went horribly awry. "We were making sparklers," he said. "Everybody was following instructions, but one boy got frustrated and started shaking the bowl and hitting it with a little masher. He said, 'Oh, this is neat.' All the other kids said, 'Stop. Don't do that.' And then it exploded.

"I just ran out of that classroom. I ran as fast as I could."

Cromer said, "That type of experiment is not one that we would use in our curriculum. The only three chemicals allowed in D.C. schools for experiments are vinegar, baking soda and alum. In the view of the D.C. schools, this would not be an experiment that would be appropriate."

A Summer Discovery brochure says the program is supposed to involve courses in earth science, mathematics, critical thinking skills, computers, aerobics, biology and astronomy. School officials said chemistry replaced the scheduled astronomy class because an astronomy instructor was not available.

Butta said chemistry teacher Lou Jagoe and a second teacher were in the lab at the time, but neither was watching the student who allegedly caused the explosion.

The blast knocked one student to the floor and sent others running down the hallways, witnesses said.

"The explosion sounded like a cannon had fired . . . . There was a big boom," said one teacher who asked not to be identified. Two persons who were on the playground said they heard the blast and saw yellow smoke billowing from the windows of the science lab.

The blast left the top of one table singed; sheets of newspaper that had covered it were turned to ashes. The sour smell of smoke filled the lab. There was no other apparent damage to the building.

Yesterday was the first day of what was expected to be a three-part chemistry experiment, said Butta, whom school system officials said is a University of Maryland professor. Butta founded the enrichment program at American University several years ago for elementary school students and introduced it to the city school system this summer.

School board member Wanda Washburn (Ward 3) said she was "concerned" about the accident, but described the program as "innovative and challenging." The program has 30 students.