584 SALTS SALTZBURG place of the two atoms of hydrogen that were contained in the sulphuric acid from which the salt was formed. The theory of the for- mation of basic salts is imperfect, and it will be observed that here there is not that com- plete replacement of basyle which exists in .neutral and acid salts. The tendency to the formation of basic salts is limited to certain acids and bases. The monad basyles do not form basic salts. The dyad metals, such as cop- per, lead, and mercury, have a strong tendency to do so, while the triads, as antimony and bis- muth, have a still stronger tendency. 4. Dou- ble Salts. In considering polybasic acids and salts, it was seen that one of the atoms of the hydrogen basyle of a dibasic acid might be re- placed by an atom of a monad metallic basyle. Such an aqid salt may be regarded as a true double salt of a metal and hydrogen. But a normal double salt maybe formed by replacing one hajf of the hydrogen basyle with one monad metal, and the other half with another monad metal. Such are called double salts, of which Rochelle salt (tartrate of potash and soda), KNaC 4 H 4 O s -I- 4 Aq, is an example. Most of the double salts have this constitution, but others have a different formation. A remark- able class of double salts was investigated by Graham. In many cases the water of crystal- lization may be expelled from a salt by the temperature of boiling water ; in other cases all but one molecule will be thus expelled, which requires a considerably higher heat. It was found that this last molecule of water could be replaced by a molecule of certain an- hydrous salts. The formation of a certain class of sulphates illustrates this action. All the sulphates of metals isomorphous with mag- nesium are capable of forming double salts of this character with some anhydrous sulphate not isomorphous with this class, as potassic sulphate. If magnesic sulphate, MgS0 4 7IIaO, which parts with six of its molecules of water at 212 and crystallizes in right rhombic prisms, and potassic sulphate, KjSO 4 , which crystal- lizes in six-sided prisms, or in four-sided right rhombic prisms, are separately dissolved in water in equivalent proportions and mingled while at a temperature a little above 212, the solution will deposit on cooling a new double salt, MgSO 4 ,KSO 4 + 6IIjO, having the same crystalline form as magnesic sulphate, but con- taining six instead of seven molecules of water of crystallization, potassic sulphate occupying the place of the seventh molecule. This sev- enth molecule has been termed by Graham saline water. Another well known variety of double salts are the alums, of which common potash alum, K,C],4SO 4 + 24H,O (or K,S0 4 , A13SO 4 + 24H,0), is an example. (See ALUM.) Haloid salts unite with each other to form double salts, the most common of which are formed by the chlorides, iodides, and bromides of the less oxidizable metals, and the alkaline and earthy metals. Examples of such double haloid salts are the double chloride of potas- sium and platinum, 2KCl,PtCl 4 , and the donble iodide of potassium and mercury, 2KI,HgI. There is a class of salts called oxychlorides, oxybromides, oxy cyanides, &c., in which one molecule of the chloride, of the bromide, or of the cyanide is united with one or more mole- cules of the oxide of the same metal, as in Turner's yellow, PbCl,,7PbO. When any acid is added to the solution of a salt the basyle of which is capable of forming a soluble salt with the radical of such added acid, a partial ex- change between the basyle of the salt and the hydrogen basyle of the added acid is supposed to take place, probably in the proportion of the relative attractions of these basyles for each radical. But if the radical of the add- ed acid is capable of forming an insoluble salt with the basyle of the salt, the latter will be entirely decomposed, and its radical appro- priated by the rauical of the added acid ; for as fast as the basic sulphate is formed it is removed from the solution by precipitation, which necessitates a continual decomposition of the first salt: Ba2NO, + H s SO 4 =BaSO 4 + 2IINO. Similar reactions take place on add- ing a base to a saline solution. If both bases and the salts which they form with the radi- cal of the salt are soluble, the solution will re- main clear ; but if the added base forms an in- soluble salt with the radical of the salt, the latter will be decomposed, while the new salt will be precipitated ; or if the base of the salt be insoluble while the added base is soluble, a soluble compound will be formed, and the base of the first salt will be precipitated. Most of the metallic salts, with the exception of those of the alkalies and the alkaline earths, are formed from bases which are insoluble in wa- ter ; consequently the addition of a soluble base, as potash or soda, to such metallic salts causes the precipitation of the base or oxide, and upon this reaction depend many of the chemical tests for metallic substances. Oxide of zinc, or zinc white, although prepared for commerce by distilling zinc into chambers sup- plied with currents of air, may be formed by precipitation from solutions of its salts by an alkaline hydrate, for instance, from the sul- phate by the action of potassic hydrate (ZnS0 4 + KaO=ZnO + KSO 4 ), potassic sulphate being formed and remaining in solution. The nature of the double decomposition which takes place when two salts are brought together depends often upon the condition in which they are. For instance, if ammonic sulphate and calcic carbonate are mixed together in a dry state and gently heated, decomposition takes place, and calcic sulphate and ammonic carbonate are produced, the latter being expelled as a vola- tile product: CaCO, + (H 4 N) a SO4=(H 4 N) 11 CO, + CaSO 4 . But if a solution of calcic sulphate and ammonic carbonate are mixed, the effects will be reversed, and calcic carbonate and am- monic sulphate will be formed, the former being precipitated, the latter held in solution. SALTZBCBG. See SALZBURG.
