The effect of a museum exhibition on art prices will be tested once again tomorrow -- another instance of how the volatile art market is affected by scholarly rediscoveries of art periods.

Eighteen paintings exhibited at the National Gallery of Art's trend-setting exhibition "The Orientalists: Delacroix to Matisse" (July 1-Oct. 28, 1984), will be up for auction at Sotheby's in New York as part of the 61-lot Coral Petroleum Inc. collection.

"The fact that the paintings were shown at the National Gallery will have an enormous effect on the sale," said Nancy Harrison, head of Sotheby's 19th-century European paintings and drawings section.

For example, one painting, "An Intercepted Correspondence, Cairo" by John Frederick Lewis, was bought by Coral at Christie's in 1979 for $451,000, at that time a record price for a Victorian painting. Sotheby's expects it to bring $1 million to $1.5 million tomorrow.

The collection was assembled by Coral chairman David B. Chalmers over a period of two to three years and many of the paintings were hung in his Houston town house, where he often entertained Near Eastern oil sheiks.

Chalmers said that selling the paintings was a "financial necessity. The oil business is not that great. Though I didn't buy the paintings with the long-range plan of selling, when it became a financial requirement, I did strategize to have the sale follow the exhibition."

Four of Coral Petroleum's subsidiaries are being reorganized under the federal bankruptcy code. The paintings in the auction are security for a $15 million loan from Banque Parbas, a French bank with American branches, said Coral's attorney, Dennis Drebsky of the New York law firm of Skadden Arps.

J. Carter Brown, director of the National Gallery, said that when the "Orientalist" exhibition was planned, "the paintings were not for sale then. Coral was riding high. We thought they might even be a corporate sponsor for us. Exhibitions take so long to get together. After we'd made this arrangement for the show, Coral went into bankruptcy. We can't make lenders sign that they won't sell paintings after we've exhibited them. We can't be oblivious of the fact that artworks in private hands can change private hands." He added, "We're not a branch of the trade."

Brown paid tribute to the art, saying Chalmers had made "a great collecting achievement, though the Coral paintings represent only a small percentage of our whole show."

The paintings, mostly from the last half of the 19th century, are full of lush colors, lustful forms, exotic landscapes and erotic fancies of North Africa and the Near East. The National Gallery show, wrote Brown in its catalogue, was "a variation and an augmentation of the show" at the Royal Academy in London March through May 1984. The two exhibitions are credited with reviving interest in the Orientalist school of painting.

The Coral collection had been scheduled to be exhibited in the Royal Academy's show but it was withdrawn, Drebsky said, because "there was concern that if the paintings left the country, they could be attached, seized, by European creditors who at that time alleged in a lawsuit now settled that a Coral company owed them money."

The National Gallery show included 84 paintings of other owners. In the catalogue, the Coral paintings were identified as belonging to a "Private Collection, Houston, Texas."

Brown said he would not have followed the recent example of the Royal Academy, which showed 53 paintings from the Florence Jay Gould collection on their way to be auctioned at Sotheby's.

In the National Gallery's early days, Brown said, it wouldn't exhibit a painting on loan from a dealer. The art dealers association threatened to organize a boycott. Later the gallery "decided that was a stuffy position. We agreed that art dealers are important and our attitude was discriminatory. Now if we have two equal choices, we do choose the art that is not for sale."

Sotheby's Harrison said she couldn't speculate on how much more the Orientalist paintings were worth following the National Gallery show. "I can't give a percentage, but the show -- art historians looking at the paintings in a serious, scholarly way -- certainly increased the prices the sale will bring. It lifts the whole tone of the sale and the price structure. After all, 350,000 people saw the show in Washington.

"It is unusual to sell at one time this many paintings from a single museum exhibition. That these were the core, the nucleus of the National Gallery show, is extremely important. There's a lot of interest on the part of museums and collectors. We sent 'Call to Prayer' by Jean Le'on Ge'ro me to an exhibit at the Bank of America's Washington branch."

Chalmers said he bought the paintings based on "economics, feel and appeal." The Coral collection was "put together over quite a short time, two or three years. I paid five to seven figures for them, so I'm not necessarily going to make a lot of money off the sale. I hope it will allow me to recover the cash I have in them, to be more liquid. I'm sure if I kept them for another five or 10 years, they would be worth more. The money will go for our debts and ongoing capital requirements. Yes, we've sold some oil wells, too."

Lewis' "A Turkish School in the Vicinity of Cairo, Egypt" is estimated at $600,000 to $800,000. Euge ne Delacroix's "Chevaux Sortant de l'Abreuvoir" is estimated to bring $300,000 to $400,000.

Considering which was the better investment: oil or oils, Chalmers said, "The right lot of oil paintings is certainly better than a dry hole."