The lights went out, the alarms went off and the weekend came early for many Northwestern students Friday.

Midterms and classes temporarily let out after a chemical spill in a research laboratory led to evacuations of at least six buildings on North Campus.

At about the same time, a tree-cutting crew caused a transformer to short out, causing power outages at Norris University Center, University Library and various off-campus buildings.

No injuries were reported in the spill, and power was restored after 38 minutes. The spill was of a chemical additive that gives natural gas its odor, not the natural gas itself.

University officials said the two incidents were unrelated.

After her modern Chinese history class evacuated the Frances Searle Building, Professor Melissa Macauley continued teaching with her class seated on a hill behind Sargent Hall. Her voice steadily rose as more alarms went off and students crowded the surrounding walkways. An ambulance drove in circles behind her and a new disruption stopped the lecture every time she tried to start back up.

“I didn’t realize this was going to turn into a comedy routine,” she said.

Across North Campus, professors and students alike waited for answers as rumors about fires and gas leaks spread.

Like many students Weinberg junior Andrea Glasauer sat on the Technological Institute’s steps, unsure if class would resume.

“I thought we’d go back in,” she said. “Now I’m just enjoying the sun.”

Shannon Schneeman arrived after the alarms went off and staked out the front of Tech to see if the building would reopen in time for her 1 p.m. midterm.

“I hope they reopen,” the Weinberg junior said. “I’d rather just get this test over with than have to wait until Monday.”

Others tried to hunt for food, but faced closures at Sargent Hall, Tech Express, Willie’s Food Court and other eateries on campus.

The spill of a beaker of mercaptan was reported at 11:14 a.m. in the Center for Nanofabrication and Molecular Self-Assembly, 2190 Campus Dr., said Capt. Kurt Dickman of Evanston Fire Department.

Mercaptan, also called thiol, is a sulfur-based chemical that is placed into natural gas to give it a strong odor.

After the beaker spilled in a third-floor laboratory, the smell wafted out of the Nanofabrication Center through its ventilation system and entered three other buildings: Tech, Ford Motor Company Engineering Design Center and Bobb-McCulloch Hall.

In addition to those buildings, Frances Searle and Sargent also were evacuated, said Alan Cubbage, vice president of university relations. University Police and EFD pulled fire alarms in the affected buildings as a precaution.

“Natural gas was shut off to all those buildings involved because we weren’t sure what we were dealing with as of yet,” Dickman said.

Cubbage said the buildings were evacuated because officials did not yet know that there was no natural gas leak.

By 12:30 p.m. people were being allowed back into the affected buildings, Dickman said.

The mercaptan spill was not much of a danger, Dickman said.

“It’s toxic but it was just a beaker full, so the amount that went out, we’re not concerned about that,” Dickman said Friday afternoon.

The Office of Research Safety, which cleaned the chemical spill, said it responded effectively to the incident.

“It was the way we’re supposed to do things,” said Susanne Wagner, the organization’s assistant director for safety. “Everyone handled things as they should, and it just turned out that it wasn’t natural gas.”

The power outage was caused by a tree branch that landed on an above-ground transformer, causing it to short out, Dickman said. A tree service working on the 600 block of Haven Street cut down the branch.

The outage occurred at 11:52 a.m. and affected 594 customers on and off campus, said Thomas Stevens, a spokesman for energy company Commonwealth Edison.

The power came back on at 12:30 p.m., Stevens said. It was the third power outage to affect NU’s campus within the last two months.

The Daily’s Aliza Appelbaum and PLAY’s Emmet Sullivan contributed to this report.

Reach Greg Hafkin at [email protected], Lee S. Ettleman at [email protected] and Elizabeth Gibson at [email protected].