[225] This holds true, it may be mentioned, with regard to the application of certain manures, such as bone-char, to the soil. Bone-char was for a long time used in France as a manure without being dissolved. The action of such a manure, containing a considerable percentage of carbonate of lime, is slower than its action would be were it pure phosphate of lime, as the carbonate of lime is first acted upon (as in the case of superphosphate manufacture) by the soil acids. [226] The solubility of tribasic phosphate, of course, is not always equal in different manures. For example, the phosphate in apatite, owing to the crystalline structure of that mineral, is not nearly so soluble as the phosphate in phosphatic guanos, although in both cases its chemical composition is practically the same. [227] For formulae of the different phosphates, see Appendix, Note I., p. 398. [228] For chemical formulae, showing reaction, see Appendix, Note II., p. 398. [229] Of course it is well known that free phosphoric acid is obtained by acting upon phosphate of lime with an excess of sulphuric acid; but the point above referred to as having been recently discovered is, that when phosphate of lime is acted upon, even by a small quantity of sulphuric acid, free phosphoric acid is formed. [230] For chemical formulae showing this reversion, see Appendix, Note III., p. 399. [231] For chemical theories on reversion of soluble phosphate by iron and alumina, see Appendix, Note IV., p. 399. [232] See Appendix, Note V., p. 400. APPENDIX TO CHAPTER XIII. NOTE I. (p. 388). The formulae, and molecular and percentage composition, of the different phosphates, are given in the following table:-- ---------------------------------------------------------------------------- | Composition in terms of-- +--------------------------+------------------- | Molecular weight. | Per cent. ---------------+-------------+-----+------+------+------+-----+------+------ | | | |Phos- | | | |Phos- Name. | Symbol. |Lime.|Water.|phoric|Total.|Lime.|Water.|phoric | | | |acid. | | | |acid. ---------------+-------------+-----+------+------+------+-----+------+------ Tri- or bone- |3CaO, | | | | | | | phosphate. | P_{2}O_{5} | 168 | 0 | 142 | 310 |54.19| 0.00| 45.81 Bi- or di- |2CaO, H_{2}O,| | | | | | | phosphate. | P_{2}O_{5} | 112 | 18 | 142 | 272 |41.18| 6.61| 52.21 Mono- or super-|CaO, 2H_{2}O,| | | | | | | phosphate. | P_{2}O_5 | 56 | 36 | 142 | 234 |23.93| 15.39| 60.68 ---------------+-------------+-----+------+------+------+-----+------+------ NOTE II. (p. 388).