a powdery substance的用法和样例：
- A letter containing a powdery substance is believed to be the source of the anthrax becteria that showed up as a skin rash on an NBC employee.
- A powdery ink used dry or suspended in a liquid to produce a photocopy.
- An in-service valve suffering from Dezincification has a white powdery substance or mineral stains on its exterior surface.
- Any of various moths whose wings and bodies have a powdery appearance.