Smyth's ozonometer, 1864.

Watercolour depicting the ozonometer installed at the top of a building. The air to be tested is drawn in from the roof by adjusting the water levels in a barrel. The test paper is held across the air stream for a fixed length of time or for a fixed volume of air. The test paper, having been treated with chemicals (such as iodide of starch) before exposure, changes colour according to the amount of ozone in the air and can be compared against a prepared scale.