Basic Mercury Salts.--The Phase Rule has also been applied by A. J. Cox[375] in an investigation of the basic salts of mercury, the result of which has been to show that, of the salts mentioned in text-books, quite a number are incorrectly stated to be chemical compounds or chemical individuals (p. 92). The investigation, which was carried out essentially in the manner described above, included the salts mentioned in the following table; and of the basic salts said to be derived from them, only those mentioned really exist. In the following table, the numbers in the second column give the minimum values of the concentration of the acid, expressed in equivalent normality, necessary for the existence of the {301} corresponding salts in contact with solution at the temperature given in the third column:-- ------------------------------------------------------------- Salt. | Normality of | Temperature. | acid. | ------------------------------------------------------------- HgCrO_{4} | 1.41 | 50° 3HgO.CrO_{3} | 2.6 × 10^{-4} | 50° | | Hg(NO_{3})_{2}.H_{2}O | 18.72 | 25° 3HgO.N_{2}O_{5} | 0.159 | 25° | | HgSO_{4} | 6.87 | 25° 3HgO.SO_{3} | 1.3 × 10^{-3} | 25° | | HgF_{2} | 1.14 | 25° | | HgNO_{3}.H_{2}O | 2.95 | 25° 5Hg_{2}O.3N_{2}O_{5}.2H_{2}O | ca. 0.293 | 25° 2Hg_{2}O.N_{2}O_{5}(?) | 0.110 | 25° 3Hg_{2}O.N_{2}O_{5}.2H_{2}O(?) | 1.7 × 10^{-3} | 25° | | Hg_{2}SO_{4} | 4.2 × 10^{-3} | 25° 2Hg_{2}O.SO_{3}.H_{2}O | 5.6 × 10^{-4} | 25° ------------------------------------------------------------- Mercuric fluoride does not form any basic salt. Since two succeeding members of a series can coexist only in contact with a solution of definite concentration, we can prepare acid solutions of definite concentration by bringing an excess of two such salts in contact with water. Indirect Determination of the Composition of the Solid Phase.--It has already been shown (p. 228) how the composition of the solid phase in a system of two components can be determined without analysis, and we shall now describe how this can be done in a system of three components.[376]