John the bartender was this lunatic who used to work at an old Center City tavern down on 15th Street. The bar was a grand oval, and at happy hour he worked it alone, like a one-man show on Broadway.

You couldn’t take your eyes off the guy, the way he performed. He’d stack his glasses in symmetrical rows, compulsively fidgeting if one seemed out of line. When he made change, he’d slam it backhanded to the bar, as if he were a matador putting the finishing touches on some poor Brahma bull.

He’d stare you in the face till you named your drink. Before you could get the “Rock” out of your mouth, he’d have his left hand on a mug and his right on the tap. Full out, the beer would fly into the mug and rise to the top like boiling lava. Then, before it overflowed, he’d slap the tap upright with his palm and slide the mug to the bar. Not a drop hit your coaster, and not a word slipped from his mouth.

Had this been a post office, you’d duck for cover whenever John showed his face. How could a guy working for tips be so serious about his job?

But you paid your 50 cents and marveled at how he could perfectly cap every mug with three-quarters of an inch of snow-like foam.

Foam, I think, is the most misunderstood part of beer. Most of us regard it as interloping, space-eating froth that prevents us from getting an honest glass of drinkable beer. That is, if we notice it at all during its thankless, all-too-brief life span.

Like John the bartender himself, foam is all show and little substance.

Unless, of course, we’re talking about nitrogen foam.

Most foam is made from the carbon dioxide that is naturally produced when yeast ferments sugar into alcohol. CO2 is added to bottles or kegs to condition the beer and keep it pressurized. Even more of the gas is used to pump beer out of the keg, through the lines and out the tap.

If you’re drinking a nice, cold American lager, CO2 gives the beer a sharp, crisp, refreshing bite. That fizzy consistency is perfect for a light-tasting beer, like Labatt’s Blue.

But many ales – especially dark, heavier bitters and stouts – already have enough bite, thanks to their hops and roasted malt content. That’s where nitrogen comes in.

Nitrogen makes smaller bubbles, and that gives beer a smoother, creamier body. The entire beer, not just the foamy head, is rich and smooth on the tongue. It’s like adding steamed milk to espresso for a silky cappuccino.

This month, Pyramid Breweries in Seattle started giving one of its beers the nitrogen kick. Called Pyramid DPA (for Draught Pale Ale), the brew will join the company’s popular unfiltered Hefeweizen as local pubs add new nitrogen lines. It’s currently available at a handful of taverns, including McGillin’s Olde Ale House (1310 Drury St., Center City) and Maggie O’Neal’s (Township Line and Pontiac roads, Drexel Hill).

When poured, DPA performs that minute-long cascade of tiny bubbles down the inside of the glass as it forms a thick, white head over copper-red ale. As with Guinness Stout, bartenders should give the beer a double pour, first filling the glass two-thirds of the way, allowing it to settle for a minute or two, then topping it off.

“It’s more mellow that way,” explained Pyramid’s Ben Meyers.

Manayunk Brewing Co. has been tinkering with nitrogen for the past month at its brewpub at Manayunk Farmers Market (Main Street and Shurs Lane). “It gives our beer a creamier lace, and it holds the head much longer,” said brewer Tom Cizauskas.

“Most American beers are over-carbonated, which is wonderful on a hot, summer day. But they have that soda pop kind of feel and they make you feel gassy,” Cizauskas continued.

He still uses CO2 on his wonderfully named Schuylkill Punch (a raspberry ale), but he’s going with nitrogen in the heavier ales and stout. “Nitrogen gives you a softer, smoother beverage, and you’re able to drink more . . . It’s a flavor thing.”

Wintertime brews

Gar Joseph, the Daily News guy who Clouts the pols, asks: “Yo, Joe Sixpack, have you written anything about winter beers yet? There’s a lot of them out there.”

Almost as many as pols, I’d say. But they come in a greater variety of flavors than just Republican and Democrat. Winter beers (or, for the secular, Christmas beers) are spiced dark brews that pack a nice wallop on a cold night.

Anchor Brewing’s Our Special Ale is a different recipe every year and goes heavy on the cinnamon, cloves or nutmeg. Celebration Ale by Sierra Nevada is especially hoppy. Snow Goose from Wild Goose and Snowball’s Chance from Blue Ridge are excellent, $25-a-case ales from Maryland; Stoudt’s Santa Claus Ale will set you back a good bit more for 25-ounce champagne bottles. I can also recommend La Binchoise Speciale Noel and Affligem Noel from Belgium – they’re both unpasteurized corked bottles that go heavy on the malt.

If you can hold on to a few bottles, they’ll age well in the cellar.

And, if you can’t hold on to them, well, join the crowd.

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