[21] As when using phenolphthalein as an indicator in neutralising by an alkali metaphosphoric acid is monobasic, and orthophosphoric acid is bibasic, it is possible by means of this difference to follow the transition of meta- into orthophosphoric acid. Sabatier (1888) carried on an investigation of this nature, and found that the rate of transformation is dependent on the temperature, and is subject to the general laws of the rate of chemical transformations which belongs to physical chemistry. Metaphosphoric acid has a particular interest in respect to the variations to which its salts are subject. The metaphosphates are formed by the ignition of the acid orthophosphates, MH_{2}PO_{4}, or MNH_{4}HPO_{4}, or of the acid pyrophosphates, M_{2}H_{2}P_{2}O_{7}, or M_{2}(NH_{4})_{2}P_{2}O_{7}, water and ammonia being given off in the process. The properties of the metaphosphates, which have a similar composition to nitrates--for instance, NaPO_{3}, or Ba(PO_{3})_{2}--vary according to the duration of the ignition to which the ortho-, or pyrophosphates from which they are prepared have been subjected. When the salts NaH_{2}PO_{4} or NH_{4}NaHPO_{4} are strongly ignited, a salt NaPO_{3} is formed, which deliquesces in the air, and gives a gelatinous precipitate with salts of the alkaline earths. But, as Graham (in 1830-40), and many others, especially Fleitmann and Henneberg (in 1840-50), and Tamman (in the nineties), observed, under other conditions the salts of the same composition acquire other properties. The above chemists recognise five polymeric forms of metaphosphates, (HPO_{3})_{_n_}. We will follow the nomenclature and researches of Fleitmann. _Monometaphosphoric acid._ The salts are distinguished for their insolubility in water; even the salts NaPO_{3}, KPO_{3}, are insoluble. They are obtained by igniting the monometallic orthophosphates--for example, RH_{2}PO_{4}--up to the temperature at which all water is evolved (316°), but not to fusion. No double salts are known.