Correction
A “correction” in the May 8 issue incorrectly identified a compound used to make rolling elements in turbopump ball bearings on America’s space shuttles as sodium nitride. We were actually closer with the original mistake, in an article in the May 1 issue that referred to silicon nitrite. In fact, the compound is silicon nitride. Silicon nitride. Silicon nitride. After discovering the second goof, our dismay only deepened with receipt of an e-mail from a PhD chemist seeking more information about the mythical material.