The acid-forming properties of the oxides of the arsenic group are maintained in their sulphides. Again, this is especially evident in the ‹higher› sulphides. The element sulphur is substituted for the closely related element oxygen without any profound change in the chemical behavior of the compounds. Advantage is taken of this acid-forming power to separate the sulphides of the arsenic group from the sulphides of the copper group, which either are not acid-forming at all, or exhibit this property only to a very slight degree.[494] «Sulpho-Salts.»—The similarity in the behavior of oxygen and sulphur derivatives, in this respect, is general and is not restricted to the metal sulphides we are discussing. For instance, the acid-forming power of carbon dioxide is shown also by carbon disulphide, the corresponding sulphur derivative of carbon. Just as the former combines with potassium hydroxide to form a carbonate, so carbon disulphide dissolves in solutions of potassium hydrosulphide and potassium sulphide to form potassium sulpho-carbonate: CO_{2} + 2 KOH ⇄ K_{2}CO_{3} + H_{2}O. (1) CS_{2} + 2 KSH ⇄ K_{2}CS_{3} + H_{2}S. (2) CS_{2} + K_{2}S ⇄ K_{2}CS_{3}. (3) [p244] The higher sulphides of the arsenic group, and arsenious and antimonious sulphides among the lower ones, combine with the alkali sulphides to form soluble alkali ‹salts› of sulpho-acids, in the same way as carbon bisulphide does. In the case of arsenious sulphide, for instance, we have the action As_{2}S_{3} + (NH_{4})_{2}S ⇄ 2 NH_{4}AsS_{2}. (4) The salt, ammonium sulpharsenite, is ionized as follows: NH_{4}AsS_{2} ⇄ NH_{4}^{+} + AsS_{2}^{−}. «Sulpho-Acids.»—The free ‹sulpho-acids› are liberated, from their salts, by any stronger acid, such as hydrochloric acid; the acids are extremely unstable and revert rapidly to the sulphides, from which their salts were originally obtained. We have, for instance, 2 HAsS_{2} ⥂ As_{2} S_{3} ↓ + H_{2}S ↑. This instability of the sulpho-acids is entirely analogous to the instability of the metal hydrosulphides (p. 203) and to the instability of certain oxygen acids, notably of carbonic acid. Sulpho-carbonic acid, H_{2}CS_{3}, is the best-known free acid of this type. It may be precipitated, undecomposed, as an oil, and its gradual decomposition into hydrogen sulphide and carbon disulphide may be observed.