The brown solution obtained when solutions of ferric chloride and potassium ferricyanide are mixed, which may be regarded as containing ferric ferricyanide, is, as is well known, very readily turned blue by reducing agents, Prussian blue or Turnbull's blue being formed. The author uses strips of filter paper dipped in the freshly prepared solution to test for traces of reducing gases, such as sulphuretted hydrogen, sulphurous acid, &c. As nitrous fumes also blue the brown solution, reducing it, traces of them can be detected by using together a piece of paper prepared as above and a piece of iodised starch paper.