A natural gas explosion demolished a town house on Capitol Hill yesterday, and the force of the blast sprayed bricks and broken glass on passers-by and automobiles.

Barbara E. Hotchkiss, 28, of Clinton, Md., who was riding in a car passing in front of the house when the explosion occurred, was hospitalized in critical condition for treatment of cuts. No one was inside the house, which was at 215 Third St. SE.

Hotchkiss, who works for Sen. John Heinz (R-Pa.), was taken to Capitol Hill Hospital and treated for multiple injuries suffered when bricks smashed through the car windows and struck her in the face.

The house, which is owned by Ronald H. Ouellet, an employe of the Department of Health, Education and Welfare, was destroyed. The blast blew out the entire front of the Old English-style structure, hurling a brass bed, furniture and curtain rods onto the sidewalk.

Lou Brott, a spokeman for Capitol Hill Hospital, said Hotchkiss was in critical condition. Her husband, Charles, 31, and their 7-month-old daughter, Patricia, who also were in the car, were treated for minor injuriesand released.

A spokesman for the Washington Gas Light Company said a preliminary investigation showed that the gas line in front of the demolished home apparently had been ruptured by construction workers who were sealing a telephone company manhole.

The spokesman said that employes of the Fort Myer Construction Co. were working with a backhoe near the gas line at the time.

Gas service to about 10 houses in the area was cut off until the extent of the leak in the line could be determined, the spokesman said.

A secretary for the Fort Myer construction firm said yesterday that no one was available for comment.

Jerry Eckholm, a fire department spokesman, said a rescue squad wagon dispatched to the scene was involved in a collision with a garbage truck. A passenger in the truck, identified as William Davids, 34, of Washington, suffered minor injuries, for which he was treated at Howard University Hospital, Eckholm said.

Most of the upper structure of the town house collapsed into the basement, ripping out huge gaps in the adjacent house at 213 Third St. SE, to which it had been attached. Then the debris ignited.

"My ears still hurt," said Harry Winter, a street vendor who was working the corner of Third and Pennsylvania when the explosion occured at 11:30 a.m. "It was so bad it knocked me up against my truck. I saw the bricks fly out. Then I saw the house come tumbling down. It was like a bomb."