[301] ‹Cf.› Geffcken, ‹Z. Phys. Chem.›, «49», 257 (1907), and the references given there. [302] ‹Cf.› Arrhenius, ‹loc. cit.›, and similar investigations on the "salt effect" (p. 109). [303] ‹Vide› Geffcken, ‹loc. cit.›, 295, and Stieglitz, ‹loc. cit.›, and p. 142. [304] See also Hill, ‹J. Am. Chem. Soc.›, «32», 1186 (1910). Hill attacks the principle as a whole, but brings no evidence against its validity for solutions of concentrations up to 0.3. [305] The limit of concentration depends, for constancy, upon the nature of the salts. The calculations, on which the data in the tables on pp. 146–7 are based, involve extrapolations which prevent the results, especially for the more concentrated solutions, being considered as final. [306] For further illustrations, ‹vide› Stieglitz, ‹J. Am. Chem. Soc.›, «30», p. 947 (1908), and the references given there to the work of Noyes, Findlay, etc. [307] Some instances are known where the solubility of a salt is ‹increased› by the addition of a salt with a common ion. In such cases it is extremely likely that an ion of the salt in question forms a ‹complex ion› with a component of the solution. ‹Vide› A. A. Noyes, ‹Z. phys. Chem.›, «6», 241 (1890), and «9», 603 (1892). In Chapter XII we shall discuss, in detail, instances of this nature where the formation of complex ions is particularly susceptible of ‹exact experimental verification›. [308] Especially by Noyes, ‹loc. cit.›, and later papers; Findlay, ‹loc. cit.› [309] This is the value for a similar ratio for KCl of the same concentration as found, by extrapolation, from the data in the table on p. 108. [310] Owing to the possibility of the formation of complex ions (Chapter XII), each individual case must be considered by itself and the most favorable conditions for the complete precipitation determined experimentally. The rule mentioned is to be used as a guide, and the reference to the possibility of the formation of complex ions considered as a warning, in the planning of such investigations. [311] ‹Cf.› p. 136, concerning precautions used to prevent precipitates from assuming the ‹colloidal› state. [312] Fresenius, ‹Quantitative Analysis›, I, 355 (1904). [313] ‹Ibid.›, I, 307. [314] Official Methods of Analysis, Bulletin 107, p. 11, U. S. Dept. of Agriculture. [315] The excess of chloroplatinic acid is first washed out of the precipitate primarily to avoid subsequent precipitation of ammonium chloroplatinate, but its removal also avoids the error discussed in the text. [316] Gay-Lussac's method. [317] Mulder. See Sutton's ‹Volumetric Analysis›, p. 304 (1904). [318] ‹Vide› Chapters XII and XIII. [319] A table of exact solubilities is given at the end of the Lab. Manual, ‹q.v.›