Invented by Marsilio Landriani in 1775, the eudiometer is an instrument for measuring the degrees of air purity. The name, coined by Landriani himself, derives from the Greek "eudios," which means "goodness of the air".

The instrument has several components: a flask with the same volume as the glass tube beneath it; a glass that can be raised by means of a screw; a small bladder; a bottle; and a cylinder with a helical spring, at whose end is attached a metal strip covered with a small leather bag filled with soft wax. This stopper is used to seal the lower end of the tube.

The bladder is filled with nitrous air by opening the stop-cock at one end of the bottle, in which a reaction takes place between metal filings and nitric acid.

The entire instrument is filled with water from the top, while the lower end of the tube is kept closed by means of the small cylinder. The bladder is placed in the upper part of the glass flask and, by opening the lower part of the tube and the stop-cocks, the nitrous air is let into the flask, while the water flows into the glass underneath through the effect of gravity. When the stop-cocks are closed and the glass is lowered, the atmospheric air enters the tube and drives out the remaining water.

The glass is then positioned so that the level of the water in it matches the zero of the graduated scale. To measure the quality of the air, the stop-cock of the flask is opened, bringing the two airs into contact. This triggers a reaction between the atmospheric air and the nitrous air, which causes a contraction in volume. Driven by atmospheric pressure and sucked in by the depression generated by the reaction, the water rises up the tube. By measuring its rise on the scale, one can evaluate the air quality.