Smoke and flame as signs of principles

Meaning

The fire-test paragraph distinguishes smoking before flame, flame without smoke, smoke without flame, and neither. It interprets these patterns through differing amounts or fixation of mercury and sulphur. Although the opening mentions all three principles, the detailed contrasts focus on these two. This paragraph does not supply an equally explicit test for salt, so its two visible signs should not be expanded into an unwritten three-part test.