There, in a drawer, sandwiched between sheets of acid-free Mylar, are two printed versions of the Declaration of Independence - what historians have called America's birth certificate.
"Both are priceless. Some on the open market have sold for millions of dollars," Arnold said as he laid the documents on a long counter. "I never get bored of this."
One was produced by printer John Dunlap the evening of July 4, 1776, in Philadelphia, and is among 26 known to exist. The other was printed about nine days later in Newport, R.I.
From the privileged few who see them - usually small groups of visitors by special request - the reaction is often the same.
They talk in hushed tones as if standing in a house of worship, Arnold said. Some tear up, get chills and goose bumps. Others take cellphone photos and quietly mouth the lofty, inspiring words:
"We hold these truths to be self-evident, that all men are created equal, that they are endowed by their Creator with certain unalienable Rights, that among these are Life, Liberty, and the pursuit of Happiness."
The visitors "have known about the Declaration of Independence since they were children," said Arnold, "but when they see it, they say, 'I can't believe this.' "
After the original handwritten Declaration was issued, about 200 copies - called broadsides - were quickly printed by Dunlap, to be posted and read aloud as a way of alerting the citizenry. "We were all in haste," John Adams later wrote. The original document is in the National Archives.
Three of the 26 Dunlap copies are in Philadelphia. The other two are held by Independence National Historical Park and the American Philosophical Society.
The National Park Service's copy - read publicly by Col. John Nixon on July 8, 1776, on what is now Independence Square - "rests" about six months a year for preservation reasons at an undisclosed location, while a facsimile is displayed in the west wing of Independence Hall.
But in the days leading up to the July Fourth anniversary, the original is placed on exhibit, as it was last week, and it will remain there through mid-December.
Two other Dunlap copies are in private hands, including one that was purchased by the television producer Norman Lear in 2000 for $8.1 million.
The "broadsides predate the signing" on Aug. 2, 1776, of the handwritten Declaration of Independence, said Karie Diethorn, chief curator of Independence Park. "It's the evidence that Congress agreed to the text and that it was to be communicated immediately to the American people that it had been adopted.
"The signed document was a formality," she said. "Dunlap's copy is so important because it was printed when the Declaration of Independence had just been agreed to."
Other out-of-state copies were printed in New York City on July 11; Newport on July 13; Boston on July 14; Exeter, N.H., on July 16; Salem, N.H., on July 20; and Charleston, S.C., on Aug. 2.
An original Newport copy - misdated June 13 - is the second declaration in the Historical Society of Pennsylvania's collection. A second edition was later printed with the correct date.
Printing errors aside, the soaring content of the document is undeniable. "There is a secular religious overtone to it," said Diethorn.
Though Thomas Jefferson didn't write it by himself, "he brought together the ideas of an era," Diethorn said. "He had a command of the English language and was incredibly skillful."
On fragile sheets of paper, the Declaration copies are a key element in retelling the nation's history. "When things are not made of precious material, we save them because they remind us of something, like your grandfather's pen and pencil set," Diethorn said. "Museums exist to preserve the corporate memory. Independence Hall is an example of that."
But at the time Dunlap and other printers were busily running off copies of the Declaration, the pages were utilitarian - not the icons they would become. Part of the society's Dunlap copy and a section of the Maryland Historical Society's copy were ripped away, possibly because paper was scarce and they were needed for other purposes.
"The Declaration of Independence is a compilation of ideas and philosophies, and once those ideas were read aloud and printed in the newspaper, the commitment was known and understood," said Diethorn. The paper it was printed on "wasn't so important because people had absorbed the ideas."
"This is a great example of how objects later become the embodiment of ideas, events, and people's lives, and that process is what makes them important - and that takes time," she said. "It's very unusual for objects to be imbued with meaning immediately," such as the twisted wreckage of the World Trade Center after the 9/11 attacks.
Opening other drawers of the safe at the Historical Society, library director Arnold pulled out a handwritten draft of the Constitution; the first photograph taken in America, showing Central High School in Philadelphia in 1839; a copy of the Emancipation Proclamation signed by Abraham Lincoln; and the diary of Tobias Lear, George Washington's secretary, where he described the last hours of Washington's life.
"Do not let my body be put into the vault in less than three days after I am dead," Washington told Lear, according to his diary. ". . . Do you understand me?" The first president wanted to make sure he was dead before he was buried.
"I feel myself going," said Washington in his last moments, according to Lear. ". . . Let me go quietly. I cannot last long."
Added Lear: "He expired without a struggle or a sigh."
Though such rare holdings are compelling, actual copies of the Declaration always seem to elicit the strongest emotions.
Visitors "from Idaho don't want to see a facsimile," said Arnold. "Some people who see it [in the vault] ask whether it's real, and I say, 'We've just gone through five locks, and you're asking me if it's real?'
"I still get a rush," he said. "I know how lucky I am to be here."
For More Information
Contact the Historical Society of Pennsylvania at 215-732-6200, Ext. 233. The vault is generally off-limits to the public, though small and large groups can set up an appointment - for a fee - through the society's programing office to
have a private viewing of some treasures
(usually about 10 items).