Americans who live in areas with substandard air won another important round last week in the tortured legal battle to force power companies, other industrial polluters and the Bush administration itself to obey the Clean Air Act. In a unanimous decision, a federal appeals court in Chicago upheld a controversial provision of the act that requires older plants to install modern pollution controls whenever they undergo physical or operational changes that increase harmful emissions... The plaintiff in the case [was] Cinergy...recently acquired by Duke Energy... Both decisions are likely to be appealed, and the Supreme Court has already agreed to hear a [similar] case... But the fact that a distinguished conservative jurist like Judge Posner [who wrote the decision] came down so clearly and sensibly on the side of the law has given clean air advocates hope that they, and the law itself, may yet prevail.