_Dimetaphosphoric acid_, on the contrary, easily forms double salts--for example, KNaP_{2}O_{6}, and also the copper potassium salt, &c. The copper salt is obtained by evaporating a solution of copper oxide in orthophosphoric acid. A blue ortho-salt, CuRHO_{4}, first separates from the solution, then a light-blue pyro-salt, Cu_{2}P_{2}O_{7}; and above 350°, when metaphosphoric acid itself begins to volatilise, the dimetaphosphate, CuP_{2}O_{6}, is formed. The residue is washed with water, and decomposed with a hot solution of sodium sulphide, when the sodium salt, Na_{2}P_{2}O_{6}, is obtained in solution. This salt, when evaporated with alcohol, gives crystals containing 2 mol. H_{2}O, which, however, retain their solubility (in 7 parts of water) after the water is driven off at 100°. When fused, these crystals give a deliquescent salt (hexa-metaphosphate). The solution of the salt has a neutral reaction, which only after prolonged boiling becomes acid, owing to the formation of orthophosphate, NaH_{2}PO_{4}. The soluble salts of dimetaphosphoric acid give the insoluble silver salt, Ag_{2}P_{2}O_{6}, with silver nitrate, and a precipitate of BaP_{2}O_{6}2H_{2}O with barium chloride. _Trimetaphosphoric acid_ is obtained as the sodium salt Na_{3}P_{3}O_{9} when any other metaphosphate of sodium is fused and _slowly_ cooled, then dissolved in a slight excess of warm water, and the resultant solution evaporated. The crystals contain 6 mol. H_{2}O, and dissolve in four parts of water. An acid reaction is only obtained, as with the preceding salt, after prolonged boiling with water. The acid is a true analogue of nitric acid, because _all its metallic salts are soluble_. _Hexametaphosphoric acid._ Fleitmann so named the ordinary metaphosphoric acid (glacial) which attracts moisture. The deliquescent sodium salt is obtained, like the trimetaphosphate, only by _rapid_ cooling. It is also formed by fusing silver oxide with an excess of phosphoric acid. The sodium salt is soluble in water, and gives viscous, elastic precipitates with salts of Ba, Ca, and Mg. Lubert (1893) obtained salts of Ag, Pb, &c. Jawein and Thillot (1889), who investigated the sodium salts of metaphosphoric acid by Raoult's method, came to the conclusion that the salts of di- and tri-metaphosphoric acid behave in such a manner that their molecule must be represented as non-polymerised NaPO_{3}, whilst those of hexametaphosphoric acid behave as (NaPO_{3})_{4}. At all events, the series of salts which Fleitmann and Henneberg regard as monometaphosphates--_i.e._ as non-polymerised--are most probably the most polymerised, because they are insoluble.