Aril 5— British authorities believe terror suspects arrested last week were planning to make a bomb that would include a highly toxic, easily obtained chemical called osmium tetroxide, ABCNEWS has learned. Used primarily in laboratories for research, osmium tetroxide is known to attack soft human tissue and could blind or kill anyone who breathed its fumes. According to the New Jersey Department of Health, it is a colorless to pale yellow solid with a strong, unpleasant odor. "It's a nasty piece of work," said Dave Siegrist, a bioterrorism expert at the Potomac Institute for Policy Studies in Arlington, Va. "It...