In the 18th century, when Philadelphia was the command center of the revolution, the Carson and Sachse families began to save important documents of their times.

The collection -- now valued at roughly $6 million -- has grown to more than 10,000 manuscripts, photographs, paintings, books, broadsides, letters and official documents. This week the Library of Congress will announce its combination purchase and gift, "the most significant acquisition of Americana by the Library of Congress in this century," said Librarian of Congress James Billington at a celebratory dinner in the Jefferson Building's Great Hall.

For many of her 92 years, Marian S. Carson, a historian, writer and expert in early American decorative arts, studied, preserved and -- mostly in the 1930s -- added to the trove begun by her ancestors and those of her late husband, John Carson.

"We're an old Philadelphia family," she modestly explained to the Chronicler. "A great deal of history has been made there."

John Henry Frederick Sachse, Mrs. Carson's great-grandfather, was a close friend of photographer Robert Cornelius. Cornelius's October 1839 self-portrait, a quarter-plate daguerreotype, is believed to be the earliest extant American portrait photograph. Sachse's son Julius acquired Cornelius's photographs and others shown at the 1876 Centennial Exhibition in Philadelphia.

Mrs. Carson has been heard to joke that her daughters, Wynne Curry and Lea Sherk, deserve credit, too, because they gave up going to the Caribbean so their mother could buy manuscripts.

"Mrs. Carson is passionately engaged in history," said Bernard Reilly, head curator of the library's prints and photographs division. He negotiated the acquisition of the very private holding. Though courted over the years by many institutions, Mrs. Carson was persuaded to transfer the collection to the Library of Congress, she said, because it alone was able to accommodate the collection's great diversity. Several thousand objects are already at the library. The rest will come over the next four years in part as a donation from Mrs. Carson and as a $2 million purchase by the James Madison Council -- a private-sector group that supports the library -- and some of its individual members.

Sixteen pieces from the collection were displayed at the council's dinner last week. The appreciative and knowing guests, including columnist Art Buchwald, an after-dinner speaker, unlike those at some other parties were much more interested in the exhibit than the hors d'oeuvres and drinks.

Reilly was ecstatic over the rarities:

A 1776 imprint of the Declaration of Independence by Joseph Loudon of New York, one of the first copies of the document. Two months or so later, Loudon escaped along with other patriots as 4,000 British troops landed. "The Toast," a 1778 drinking song honoring George Washington, handwritten by its composer, Frances Hopkinson, a Declaration signer.

An 1800 portrait by Charles Saint-Memin (1770-1852) of Washington with a laurel victory crown, reflecting the classical influence on the first president's life. A life mask of Washington (who died in 1799) is believed to have been the model for the black-and-white chalk on paper drawing.

Two printed reports on legal cases signed by Samuel Chase (1796-1811) -- the only U.S. Supreme Court justice to be impeached by the House and tried by the Senate. He was accused of "intemperate charges to juries." Reilly notes that Chase, a "strident critic of Thomas Jefferson," was not convicted. A rare 1856 watercolor depicting the senate of the young African nation of Liberia, settled in mid-century by former American slaves. Documents pertaining to the nation's founding, including the first rules and regulations on the judiciary, education, science, medicine and technology; papers on women's history, Free Quakers and Moravian music, and a self-portrait by painter Gilbert Stuart and letters of John Adams, James Madison and James Monroe.

Then as now, many letters from American political figures ask for or promise money. Three letters show how hard up for money the revolutionists were.

Among the famous correspondents are Thomas Jefferson and John Hancock -- the two-time president of the Continental Congress and the first signer of the Declaration, as well as other signers Francis Lightfoot Lee, Benjamin Harrison and Richard Henry Lee together with Joseph Jones, all members of the Virginia congressional delegation. In this election year, it's timely to note the first presidential campaign biography. John Beckley, the first Librarian of Congress, wrote "Address to the People of the United States With an Epitome and Vindication of the Public Life and Character of Thomas Jefferson." It was published during the 1800 campaign to "counter attacks on Jefferson's character which appeared in newspapers during the bitter election campaign," said Billington. He added with relief, "Fortunately, the duty of writing campaign biographies no longer falls to the Librarian of Congress."

Now only scholars can study the portion of the Carson Collection already transferred. All others must restrain their curiosity until next May when the "Treasures of the Library of Congress" exhibit marks the grand reopening of the restored Thomas Jefferson Building. The exhibit will also be shown on the National Digital Library Program's Web site: http://www.loc.gov CAPTION: Librarian of Congress James Billington and Marian S. Carson admire a portrait of Washington that the library is acquiring from the Carson family of Philadelphia. CAPTION: A daguerreotype from 1839, Robert Cornelius's self-portrait, is part of the historical trove.