On January 15, 1855, Thomas Sessford, 36, achieved the melancholy distinction of becoming the first patient admitted to the Government Hospital for the Insane, now known as St. Elizabeths Hospital.

Sessford had the brand-new institution to himself for a week. An escapee from a Maryland insane asylum, he was described as "temperate and of good moral character," but heir to "insanity (that) manifested itself in violence in his Mother's family," and was put away because he "fancies a young lady is in love with him."

It was hoped that Sessford would be the first of many to benefit from what was then, and remains yet, a noble experiment: humane handling of the mentally ill, designed to restore them to a full and free life. The usual "treatment" of the time was to lock crazy people up and try to beat the devils out of them or neglect them until they died.

Because it was recognized that people's surroundings have a lot to do with how they feel, humanitarian Dorothea Dix persuaded farmer Thomas Blagden to let go of 300 wooded acres across the Anacostia, commanding a splendid view of Washington, for the hospital grounds. The buildings, begun in 1852, were of generous and pleasing proportions, high-ceilinged and as light and airy as such massive construction could be.

But if the attitude at the hospital was enlightened, the medical practice of the period was not: The cause of Sessford's death the following August was listed as masturbation. He also was the first patient to die in the federal government's first mental hospital.

This sort of information just tumbles out of Wilhelmina C. Carey and Josephine Hart Thrasher, who have spent the past couple of years inventing what this weekend becomes Washington's newest museum, located in the splendid old superintendant's quarters at St. Elizabeths.

The idea for the museum grew out of an inventory Carey, then head housekeeper, began several years ago. "Pretty soon I had catalogued nearly 500 fine old pieces and more was turning up every day." Thrasher, a professional appraiser, was called in to help and has been there ever since. Now more than 1,700 items have been catalogued, and more are still turning up every day.

"I was going through the old Superintendant's Office and opened a closet," Thrasher said. "It was full of fine Meissen china, including four tea services. After that was all catalogued I went on to the next closet. It was full of Wedgwood china."

The china and hundreds of other valuables had stood untouched since 1962, when a new superintendent declined to occupy the grand suite, recalled psychiatrist E.R. Vann, "because the government had decided to start charging rent; at fair market value that would have eaten up most of the salary." It speaks for St. Elizabeths' institutional integrity, and the care of housekeeper Joyce J. Flanary, that the Civil War-period and Victorian-era treasures still were there when the inventory was undertaken.

It also has to do with institutional continuity: Carey went there 22 years ago as a nursing student and never has seriously considered leaving. "This is not one of those government places where people come and go," she said. "People come here and stay. We care about St. Elizabeths. We're proud to work here." Part of that staunch loyalty stems from defensiveness: they work at a "nut house" the press and public ignore except when something goes wrong.

Carey is as mindful of the properties as if they were her own, and this proprietary passion is shared by Thrasher and the craftsmen, including Vernon Colbert and Orlando Parks, who have labored long and late to get the museum ready for Friday's opening. St. Elizabeths had little money to spare for the project, so most overtime has been voluntary and all restoration has been done in the institution's workshops.

Do-it-yourself has always been the rule at the hospital, whose original buildings are made of timber cleared from the knoll and bricks fired of clay dug for the foundations. The walls of Center Building, first to go up, are 22 inches of solid masonry, and the flooring is still so sound that a knife cannot be inserted between the boards.

The museum, even as seen through the dust and disarray of last-minute remodeling, is a marvel. Miss Dix would find her desk just as she left it, and still no doubt would grouse that her official portrait was too flattering. Old Joseph Henry, of Smithsonian Institution fame, would find the boardroom virtually unchanged from when he sat as an orginal St. Elizabeths trustee. Abe Lincoln would find the same bed he used to rest on after riding out from the White House, and it would still be far too short.

Pvt. Charles Schroeder of Company B, 41st New York Volunteer Infantry, would be proud to see on the wall his penciled allegorical drawing of the hospital, done while he was recovering from the horrors of the Civil War. The drawing's legend charges us all to Love they Neighbor as they selfe.

The museum will hold open house from 1 to 4 today. Thereafter, visits may be arranged by calling Mrs. Carey at 574-7700. ST. ELIZABETHS HOSPITAL MUSEUM -- Center Building, 2700 Martin Luther King Jr. Avenue SE.