Crowds of angry blacks marched through parts of Pittsburgh last week, exchanging volleys of stories, bricks and bottles with white workers at the sites of unfinished buildings. Determined to get a fair chance at construction work, the demonstrators tied up traffic as they converged on a new 65-story U.S. Steel skyscraper and scuffled with club-wielding police, who arrested more than 200 of them. The skirmishes introduced an ominous novelty into urban racial disturbances: demonstrators and police squirted each other with disabling Chemical Mace. Forty-one people were injured, eleven seriously enough to require...