Scientists at Los Alamos National Laboratory in New Mexico are looking for old air. They are inviting people who have some to send it in.

The scientists are trying to figure out whether the amount of carbon dioxide in the air today is appreciably different from centuries ago, and they need some old air to make the comparison.

It is known that the level has been rising slowly since measurements began in recent years, a finding that has led to forecasts of a global warming trend because carbon dioxide gas lets sunlight reach the Earth's surface but blocks heat from radiating back into space.

There are still questions, however, about whether the rise is merely part of a long-term cycle of rising and falling levels and whether the Earth has sustained high carbon dioxide levels in the historical past.

To answer these questions, scientists need to test air from long ago. It occurred to Los Alamos scientists that samples of ancient atmospheres would have been captured inside hollow but sealed objects made centuries ago. As a result, they are looking for objects such as old brass buttons, antique hour glasses, and old telescopes that were made at known dates in the past and that have airtight seals.

Some old air troves have already been found. Chicago's Adler Planetarium has a 17th-century hour glass that will be opened and its air sampled. A sunken Mississippi river boat is expected to contain sealed items when it is raised. One source, old cremation urns from a Buddhist temple in Hawaii, turned out to lack good seals.

People who wish to donate old air should first write the research leader, Ernie Bryant, at Los Alamos National Laboratory, Los Alamos, N.M. 87545.