Thio-compounds are known, analogous to these salts. Mention may be made of mixed oxy-thio-phosphates, e.g. Na₃PO₃S and Na₃POS₃, which result from the action of alkaline hydroxides on phosphorus penta-sulphide, a grey solid produced by direct combination of phosphorus with sulphur. They are easily decomposed by hot water; hence the thio-arsenates and the thio-antimonates are better known. Arsenious sulphide, As₂S₈, a yellow precipitate formed by the action of hydrogen sulphide on a solution of arsenious chloride, and antimony trisulphide, Sb₉S₃, an orange precipitate similarly formed, dissolve in solutions of poly-sulphides of t he alkalies; these solutions, on evaporation, deposit crystals on cooling : As₂S₃ = 2K₂Si.Aq = 2K₃AsS₄.Aq + S_(n_₂₎. Sodium thio-antimonate, Na₃SbS₄.9H₂O, has long been known as "Schlippe's salt." One of the sulphur atoms may be replaced by selenium, giving Na₃SbS₃Se-9H₂O, thus showing the similarity in character between sulphur and selenium.