To produce oxygen by Priestley's method
Place some red mercuric oxide in a small test tube and heat for several minutes. Hold a glowing splint at the mouth of the tube. Note what happens to the splint and watch the sides of the test tube.
You will notice that the glowing splint is relighted and tiny grey beads are produced on the sides of the test tube.
The gas that relights a glowing splint is the same gas obtained by Priestley when he heated red oxide of mercury. It is oxygen. The grey beads are beads of mercury.