The loss of phosphoric acid incurred by the present method of sewage disposal is not so large as the loss of nitrogen, inasmuch as the quantity of phosphoric acid contained in human excreta is very much less. Roughly speaking, it may be said to amount to a little less than one-third of the nitrogen lost in this way. _Sources of Artificial Gain of Phosphoric Acid._ To balance these losses, we have a practically unlimited supply of mineral phosphates for application as artificial manure, as well as large quantities of other manures, many of them already mentioned in connection with nitrogen, such as bones and guanos of all kinds. Quite recently, also, a large source of phosphoric acid has been opened up in the basic slag, a rich phosphatic bye-product obtained in considerable quantity in steel-works from the basic process of steel manufacture. We have also large quantities of phosphoric acid in the imported feeding-stuffs, for statistics regarding which we would refer our readers to a previous chapter. The question of the actual amount contained in these sources is not of the same interest as in the case of nitrogen, and need not therefore detain us. We have sufficiently indicated the importance of phosphoric acid in agriculture by the statements above given. All further consideration of phosphoric acid must therefore be deferred to future chapters. FOOTNOTES: [123] See Appendix, Note I., p. 210. [124] See Appendix, Note II., p. 211. [125] These results, as indeed all soil percentages, are calculated on the soil in a dry condition. APPENDIX TO CHAPTER V NOTE I. (p. 201). COMPOSITION OF APATITE (Voelcker). (_Krageroee, Norway._) Lime 52.16 Phosphoric acid 41.25 Chlorine 4.10 Fluorine 1.23 Oxide of iron 0.29 Alumina 0.38 Potash and soda 0.17 Water 0.42 ------ 100.00 ====== Apatite is found in considerable quantities in America, Germany, France, Spain, Hungary, Norway, and Great Britain. According to Rose, apatite is made up of three molecules of tribasic calcium phosphate (Ca(PO_4)_2), combined with one molecule of calcium fluoride (Ca F_2) or one molecule of calcium chloride (CaCl_2) respectively. The composition of the pure mineral should be-- _Chlorapatite._ Per cent. Calcium phosphate 89.38 Calcium chloride 10.62 _Fluorapatite._ Calcium phosphate 92.31 Calcium fluoride 7.69 NOTE II. (p. 203). The following is a list of the commoner rocks in which the percentage of phosphoric acid has been determined. The results are taken from analyses by Nesbit, Schramm, Bergemann, Rose, Deherain, Handtke, Petersen, Nessler, Muth, Fleischmann, Storer, and others:--