_Bismuthous oxide_, Bi_{2}O_{3}, is a feeble and unenergetic base. The normal hydroxide of the oxide Bi_{2}O_{3} is Bi(OH)_{3}; it parts with water and forms a metahydroxide (bismuthyl hydroxide), BiO(OH). Both of these hydroxides have their corresponding saline compounds of the composition BiX_{3} and BiOX. And the form BiOX is nothing else but the type of the basic salt, because 3ROX = RX + R_{2}O_{3}. It is evident that in the type BiX_{3} the bismuth replaces three atoms of hydrogen. And indeed with phosphoric acid solutions of the bismuthous salts give a precipitate of the composition BiPO_{4}. On the other hand, in the form of compounds BiOX or Bi(OH)_{2}X, the univalent group (BiO) or (BiH_{2}O_{2}) is combined with X. Many bismuth salts are formed according to the type BiOX. For instance the carbonate, (BiO)_{2}CO_{3}, which corresponds with the other carbonates M_{2}CO_{3}. It is obtained as a white precipitate when a solution of sodium carbonate is added to a solution of a bismuth salt.[45] The compound radicle BiO is not a special natural grouping, as it was formerly represented to be; it is simply a mode of expression for showing the relation between the compound in question and the compounds of other oxides. [45] Basic bismuth carbonate is employed for whitening the skin (veloutine, &c.) Three _salts of nitric acid_ are known containing bismuthous oxide. If metallic bismuth or its oxide be dissolved in nitric acid, it forms a colourless transparent solution containing a salt which separates in large transparent crystals containing Bi(NO_{3})_{3},5H_{2}O. When heated at 80° these crystals melt in their water of crystallisation, and in so doing lose a portion of their nitric acid together with water, forming a salt whose empirical formula is Bi_{2}N_{2}H_{2}O_{9}. If the preceding salt belongs to the type BiX_{3}, this one should belong to the form BiOX, because it = BiO(NO_{3}) + Bi(H_{2}O_{2})(NO_{3}). This salt may be heated to 150° without change. When the first colourless crystalline salt dissolves in water _it is decomposed_. There is no decomposition if an excess of acid be added to the water--that is to say, the salt is able to exist in an acid solution without decomposing, without separation of the so-called basic salt--but by itself it cannot be kept in solution; water decomposes this salt, acting on it like an alkali. In other words the basic properties of bismuthic oxide are so feeble that even water acts by taking up a portion of the acid from it. Here we see one of the most striking facts, long since observed, confirming that action of water on salts about which we have spoken in Chapter X. and elsewhere. This action on water may be expressed thus:--BiX_{3} + 2H_{2}O = Bi(OH)_{2}X + 2XH. A salt of the type Bi(OH)_{2}X is obtained in the precipitate. But if the quantity of acid, HX, be increased, the salt BiX_{3} is again formed and passes into solution. The quantity of the salt BiOX which passes into