The two candidates for Congress in Northern Virginia's 8th District disagreed on almost everything last night in their second debate of the campaign, and elicited howls and applause by deriding each other.

John F. Herrity, the Republican challenger, referred claims made by his opponent, Democratic incumbent Rep. Herbert E. Harris II as "hot air" and declared: "Figures lie and liars figure."

Harris accused Herrity, who is chairman of the Fairfax County Board of Supervisors, of missing 265 board votes since becoming chairman. Harris claimed that he, as opposed to Herrity, is not a man to "duck the tough issues."

About the only thing that herrity and harris could agree on at the hour-long debate sponsored by the North Springfield Civic Association was college tuition credits.

Admitting that their children and their pocketbooks would benefit, both said they favored federal aid to help parents pay for their children's college education.

Herrity, announcing that inflation is eating away at the "very soul of the country," said, as he has before, that he supports a constitutional amendment requiring a balanced federal budget.

Harris, as he has before, called the balanced budget amendment unrealistic and said the answer to economic problems is an expanding economy.

Both candidates admitted that medical costs are too high. Harris said, "You've got the little guy going to the hospital and saying, 'What will I do?'" The answer, Harris said, is national health insurance.

The answer, Herrity said, could not be national health insurance because such a program would "eliminate forever and ever" the chance for a balanced federal budget.

"If you like the postal service, herrity said, "you'll love national health insurance."

Harris, a two-term incumbent, started off last night in the crowded auditorium of the Washington Gas Light Co. building in Springfield by asking himself, "What have you done, Mr. Harris?"

Harris said he had done plenty, including his successful effort to block a proposal in the House for a Northern Virginia commuter tax Harris said he has brought federal jobs to Northern Virginia and that he has a 99.2 percent attendance record for House votes. "I'd call that an A plus," Harris said.

The congressman attacked Herrity's attendance record for voting in the Fairfax County board. He said Herrity had a 74 percent record, a "Dminus."

That's when Herrity said "liar figure and figures lie." Herrity announced his own figures showing that his attendance record was 81.1 percent, and he argued that his record record is better than Harris' was when harris served on the board of Supervisors. That record was 79 percent, Herrity said.