Ah, that elusive movie magic known as chemistry. What is it? How does it work? Is it a happy accident, or can it be contrived?

Maybe it's just another variation on the cliche, "I don't know much about art, but I know what I like."

In other words, cinematic chemistry between actors is intangible. Often, it is hoped for, and when it is achieved, it is sometimes pursued again in the hope of lightning striking twice. And sometimes it does.

For the purposes of this discussion, I'm not talking about comic chemistry, such as Laurel & Hardy or Abbott & Costello had in abundance, or which was later demonstrated by the Monty Python crew or Bill Murray and Dan Aykroyd in the "Ghostbusters" films.

We're talking romantic chemistry here. When an actor and actress are put together by fate or by casting and somehow convince an audience for two hours that they are truly in love and that it will last forever.

And in a way, that modern romantic comedies, or "rom-coms" as they are now called, seem to have abandoned in favor of crude sex gags and the pursuit of one more conquest instead of true love.

In the past, such chemistry was essential in the best romantic comedies (and dramas).

Take Myrna Loy and William Powell, who clicked so well they were teamed in no fewer than 14 films (including six in the "Thin Man" franchise), or Katharine Hepburn and Spencer Tracy, who appeared together in nine.

Even Barbra Streisand and Ryan O'Neal showed us that chemistry can lift a good picture, as with the sublime farce "What's Up, Doc?" and even bolster a failure, i.e. "The Main Event."

Julia Roberts and Richard Gere had it in both "Pretty Woman" and "Runaway Bride." Roberts also sparkled with George Clooney in the "Oceans" trilogy. As did Diane Keaton with Woody Allen in a string of comedies, some more romantic than others, in the 1970s.

Or even better, Tom Hanks and Meg Ryan, first in the underrated "Joe Versus the Volcano," then in the hits "Sleepless in Seattle" and "You've Got Mail."

But recently, we've had way too many sad examples of the reverse. After "The Tourist," it's a pretty good bet that we won't be seeing Johnny Depp and Angelina Jolie paired in another movie very soon. To the surprise of many, there was no spark whatsoever.

Similarly, "How Do You Know" tried to put together Reese Witherspoon and Paul Rudd, two witty players who have been funny and charming in other vehicles but together couldn't seem to muster anything approaching a romantic tingle. (Oddly, the deadhead third member of the film's triangle, the insensitive, womanizing jock played by Owen Wilson, became the film's most interesting character.)

So where is romantic movie chemistry in contemporary rom-coms?

Hugh Grant was successfully paired with Sandra Bullock in "Two Weeks Notice," Drew Barrymore in "Music and Lyrics" and Julia Roberts in "Notting Hill" — but just one time with each co-star. And with the wrong partner, he can be at sea, as demonstrated with Sarah Jessica Parker in "Did You Hear About the Morgans?"

Sandra Bullock and Drew Barrymore have each had great success in other romantic comedies, but they also have never been matched up with the same co-star twice. (Bullock has made two films with Keanu Reeves, but neither was a comedy. And Barrymore has played opposite Adam Sandler twice, but those were gimmick farces, not rom-coms ... and was that really chemistry between them?)

Of course, there were a lot of old-school actors who also played opposite co-stars only once, yet seemed to have remarkable chemistry with each and every one. Audrey Hepburn, for example (with Cary Grant, Gregory Peck, Humphrey Bogart, Fred Astaire, Gary Cooper).

Others, such as Doris Day, were paired in romantic comedies with co-stars on one occasion (Frank Sinatra, Cary Grant, David Niven, Jack Lemmon, Clark Gable, Gordon MacRae) and others more than once (Rock Hudson, James Garner and Rod Taylor), never failing to connect.

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But among the current crop of younger stars we associate with rom-coms — Jennifer Aniston, Kate Hudson, Katherine Heigl, Kristin Bell, Jennifer Garner — there is a remarkable number of bad comedies. Did they have any real chemistry with any of their co-stars? Hard to tell.

And recent stabs at the genre by such seasoned vets as Michelle Pfeiffer and Meg Ryan have bypassed theaters and gone directly to DVD. Not a good sign.

Maybe it's as simple as this: Since Hollywood no longer seems capable of making sweet romantic movies, comic or otherwise, chemistry isn't even a consideration.

e-mail: hicks@desnews.com