Josip Franjo (or: Josephus Franciscus) Domin (1754-1819) was a Croatian physicist, affiliated at the Universities of Gyor and Pest, Hungary. He is noted for his book Physical Treatise on the Genesis, Nature and Utility of Factitious Air, written in Latin (Dissertatio physica de aeris factitii genesi, natura, et utilitatibus) and published in Gyor in 1784. In his Treatise he described various kinds of "artificial air", discovered mostly by Joseph Priestly, as well as methods and devices for their preparation, and - in the second part of the book - their usage. This paper discusses his description of the eudiometer, an apparatus for measurement of oxygen ("dephlogisticated air") content of gas mixtures by its reaction with "nitrous air" (nitrogen oxide, NO), invented by Priestley and improved by Felice Fontana. Especially discussed was the need for measuring the "goodness of air" in relation to the phlogiston and miasma theories, as presented in the Treatise.