Old-fashioned seltzer, the drink that never goes flat, is now almost entirely unavailable in the classic glass siphon bottles with metal spigots, but it has a self-appointed savior: Walter Backerman, the last full-time, person-to-person deliveryman in Manhattan of the drink.

Mr. Backerman is 52, with a trimmed mustache and Popeye-like forearms. One midsummer morning, he set out at five o’clock, as usual, on his daily route. “I’m rescuing seltzer from extinction,” he said. “I’m the last of my breed. That stuff they call seltzer in a can doesn’t have half the fizz. It’s flat the second you open it.”

Mr. Backerman was wearing a red and black Craftsman Tools ball cap, jean shorts, steel-toed boots, and a fanny pack clipped around his shirttail with a cell phone on the belt. He got into a 1984 Grumman step van parked in his Queens Village driveway, then rattled over the 59th Street Bridge heading for his first delivery.

“In 1919, my grandfather, Jacob Rosenbloom, started on this same route in his horse-drawn wagon from a stable on Broome Street, and my father, Al Backerman, used to go the same way in his red International Harvester LoadStar 1600 truck,” he said. “They filled up like I had to do yesterday. They had lots of shops to choose from. Now the only place to fill, where I went, is Gomberg Seltzer Works in Canarsie, from the same kind of machine, the Monitor, a 1910 Barnett and Foster siphon filler. It cools triple-filtered water to 38 degrees so it can be forced, carbonated, into the bottles.

“I began delivering at the age of 10, working and going to school, and always staying loyal to seltzer,” Mr. Backerman continued. “The other guys delivering in Manhattan bugged out. They just left their routes in the streets. One guy handed me his route cards and went to work for Home Depot. I can’t desert my seltzer drinkers. They call me ‘Mr. Bubbles.'”

He stopped at a light and, without getting out of the truck, grabbed a coffee regular and a cheese Danish from a pushcart. He started the truck with a lurch and pointed a thumb behind him.

“I’ve got three metal shelves back there riveted to the cargo-bay floor, with a few empty seltzer boxes and 30 full ones,” Mr. Backerman said. “My seltzer boxes were handcrafted from red oak pallets by my friend Alex Katz. Alex passed away last April. He was my customer, too. Every box took him six hours to make. I got over a thousand boxes from Alex. I traded him 20 bottles of seltzer for each box. I can carry a full box on my shoulder. Seventy-five pounds a box!

“I’m the only one doing Manhattan who has the old bottles,” he went on. “There were millions of them in the ’20s, seltzer’s golden era, with hundreds of seltzer men. Now the majority of what remains is my 15,000 beauties – all over 50 years old! Those priceless gems from my father and seltzer men I knew as a kid. Most are Bohemian glass, the only kind my Pop would buy – they’re half-an-inch thick, cobalt, green, or iridescent clear glass, and were hand blown into a form. Most glassworks that made these were destroyed during World War II. They’re irreplaceable. I know every bottle on my route. My rare Bohemian seltzer bottles can be worth up to 200 bucks each. You can get imitation seltzer bottles from places in Connecticut and San Francisco, but the heads are plastic. I won’t use them. They’re a corruption of the form.”

The truck bucked, the bottles clattering. “I know my seltzer history,” Mr. Backerman went on, “The siphon bottle was invented in 1837 in France. Carbonated water was meant to duplicate water from naturally occurring effervescent mineral springs. It was a big deal in Europe, a passion that the immigrants brought with them to New York.

“Two entrepreneurs on the cutting edge of this technology imported French siphons in 1861, and had a huge factory – at 433-444 First Ave., now NYU Medical School,” he said. “One of them, Carl H. Schultz, opened the Central Park Mineral Springs, a gigantic fountain that served seltzer from 5 a.m. to 9 p.m. daily, near the Dakota. Seltzer made Schultz a millionaire.”

First stop: a loft in Chelsea. Mr. Backerman grabbed a dolly from his truck, loaded it with a couple of Alex Katz boxes, and wheeled it into the elevator. A young woman in a T-shirt and shorts, holding the hand of a 4-year-old girl, greeted him. With hugs.

“Seltzerman!” they both yelled, the little girl hopping up and down. Mr. Backerman handed them a flyer explaining a price increase to $2.50 a bottle, from $2.

“Fine,” the woman said, “After all, my husband’s grandparents took seltzer from your Dad, right?”

“Right,” Mr. Backerman said.

“It’s a party in my mouth,” the little girl said gravely.

Back at the truck, Mr. Backerman squirted some of his own seltzer into a Nathan’s paper cup and took a swig.

“I even drink it warm,” he said, “Some joker slipped me a mickey of store-bought soda the other day. It was vile! Disgusting!”

Next stop: across town, East 14th Street. Mr. Backerman heaved a case of seltzer onto his shoulder and climbed five flights. “This is reality,” he said, winded a little. The door was open. “Seltzerman,” Mr. Backerman shouted. An elderly woman in a pink housecoat and matching slippers appeared. Mr. Backerman handed her a flier and apologized for the price increase.

“I’ll pay it. I’ve been taking seltzer in this apartment for 57 years,” the woman said, “My parents always took it. The taste of seltzer out of the old bottle reminds me of being a little girl. We stick with seltzer, whether it’s a nickel a bottle or whatever. It’s my medicine.”

More stops: Eldridge Street Synagogue; then another 20 blocks to Celeste the make-up artist; two podiatrists on East 79th. Then a 30-minute trek to a Linda Herskowitz in Teaneck, N.J. In her driveway, Mr. Backerman set four cases of seltzer by the garage door, just as a chic young woman pulled up in a black Mercedes sedan and leapt out. “Walter! Am I glad to see you,” Ms. Herskowitz said, “My 19-year-old, Noah, is enlisting in the U.S. Army next month. For basic training he had to give up Cokes. He’s on seltzer now.”

Last stop: 6 p.m., Riverdale in the Bronx. “My oldest customer,” Mr. Backerman said, “Mildred Blitz. She’s 83.” A small, gray-haired woman rolled up on a chair with four caster wheels.

“Your father delivered seltzer to me the day you were born,” she said cheerfully to Mr. Backerman. “You want some cold seltzer?” They toasted each other. “I just got back from visiting my daughter in Denver.” she said, “She got me a national brand of seltzer in the can. I took one sip and gave it to the dog.”