A Springbrook High School senior is studying the atmosphere of the planet Jupiter by mixing the main gases that are found there with a spark of electricity and keeping track of the compounds they produce.

Another senior, from West Springfield High School, is studying a small marine worm, called a nematode, and charting how it moves in the sand, according to how the tide change.

Arjun Yodh of Silver Spring, who is studying Jupiter, and Nancy Zeleniak, who is studying nematodes, are two of the 40 finalists nation-wide in the annual Westinghouse Science Talent Search, which has been trying to find the nation's most promising future scientist for the past 36 years.

There are two other students from the Washington area who are part of the finalist group, which is gathering for four days at Washington's Mayflower Hotel.

Lawrence Weatherford, who also goes to West Springfield High in Fairfax County, is doing experiments on digestive enzymes. Glenn Poole, a senior at Annandale High School, has written a computer program that tries to show the underlying structure of English sentences so that computers can be told what to do in simple English rather than in a special computer language.

According to the scientists who judge their work, the high school contestants often are doing work that is as difficult and creative as the research projects that earn a doctorate degree.

"I ask some of them the same type of questions that I would ask at a Ph. D. exam," said Glenn T. Seaborg, a Nobel Prize winner from the University of california, who has been on the panel of judges for 15 years. "The very best of them do quite well. We're looking for the ones with the most potential as a contributor to human welfare through science, and I think they seem to be getting better every year."

The finalists this year were chosen from among 13,000 applications from 49 states. But 14 of the 40 finalists come from New York state, and seven are from one high school, the Bronx High School of Science, which has more finalists than any of the 19 states represented, except it own.

At a black-tie dinner Tuesday night the judges will announce who they think is the best of the lot, an honor that carries a $10,000 scholarship. Nine other big scholarships will also be awarded, but the students who have come to Washington seen as interested in meeting each other as in winning the top prizes.

"It's incredible," said Christopher Loshe, who comes from Cocoa, Fla. "I feel just like a sponge in the middle of all this knowledge, absorbing what's around here."

"When you read all their biographies, you think everybody is a superstar," said Arjun Yodh, "but when you meet them you find out that they're pretty normal."

But the camaraderie among the finalists at the Mayflower is much different from the situation in most of their high schools, where they say bright students are often outsiders. The exception is Bronx High School of Science, which picks its students from throughout New York City on the basis of a competitive examination.

"It's the atheletes that are dominant in my school," said Yodh, whose father, a physicist, comes from India. "You're kind of secluded when you're doing scientific work. Among most of the students it's all football and basketball . . . Nobosy really knew what I'd been doing until I became a finalist in this, and then everybody's rooting for you because you're from their school."

Nancy Zeleniak, whose father is a machinist, said she "just can't wait to get out of school every day, and get to work. You know, I'm really not interested in school any more at all."

Her work, since last fall, has been at the Smithsonian Museum of Natural History, organizing her research data on nematodes.

"Being with the scientists is like a whole new way of life," she said. "They don't care what you wear. They don't care what you like. They care about what you do and what you know, and everybody is willing to listen. I've even had a doctor of science working under me."

Dorothy Schriver, who has organized the contests since they began in 1942, said that over the years, the outlook and values of the finalists had changed very little despite major changes in society.

"They're extremely bright and extremely motivated, and they believe in the great need for scientific advancement," she said.

The contest is based on merit only, Mrs. Schriver said.

"That may seem almost old-fashioned," she said, "But I think it is very important that we keep it that way. Don't misunderstand me, I'm all in favor of doing things for people who are disadvantaged. But I think we have a reward our achievers too."