NORTHUMBERLAND - A child visiting the Joseph Priestley House in Northumberland once asked, "What did people breathe before Dr. Priestley discovered oxygen?"

The answer, of course, is that air already contained oxygen, which the English scientist isolated and identified while doing experiments in his laboratory in 1774. Priestley also was the first to observe that plants produce oxygen that we breathe.

Some 240 years have passed since Priestley's accomplishment. In recognition, Oxygen Day will be observed Sunday at the Joseph Priestley House, 472 Priestley Ave., Northumberland. A living history interpreter portraying Priestley will present free programs on Priestley's discovery.

Priestley, a renowned scientist, political thinker and dissenting clergyman, lived in Northumberland from 1794 until his death in 1804. Best known for discovering oxygen, Priestley is credited with isolating eight gases.

Oxygen Day is sponsored by the Friends of the Joseph Priestley House. Priestley's program will be conducted twice - at 1:30 and 3 p.m. - in the Pond Building on the grounds. The program is suited for all ages, especially youngsters interested in science.

Portraying Priestley will be Ronald Blatchley, a retired chemistry teacher who has been interpreting the British scientist for more than 30 years and who has given presentations on Priestley in the U.S. and England. He will explain and demonstrate chemistry discoveries of the 18th century.

Priestley House will be open from 1 to 4 p.m. with free admission. Visitors may tour the late 18th century Georgian house at their own pace and visit with guides wearing 18th century dress who will greet them and answer questions.

More information is available by calling 570-473-9474 or by going to www.josephpriestleyhouse.org on the Internet.

The Friends of Joseph Priestley House is a not-for-profit membership organization established in partnership with the Pennsylvania Historical and Museum Commission. Its purposes are to promote interest in Joseph Priestley and his work and to research and restore the Joseph Priestley House. To accomplish these purposes, the Friends provide educational programming and conduct fundraising activities.

Members receive free admission to Joseph Priestley House, discounted rates at special events and 10 percent discounts on purchases made at the gift center. The Friends are always seeking additional volunteers to staff the visitor's center and conduct tours of the Priestley House Museum.