The ash which is left on burning bones used to be an article of considerable manurial importance. It is still imported from South America in some quantity, and is used chiefly in the pottery industry. It is occasionally still used to a limited extent for the manufacture of high-class superphosphates. It is extremely rich in phosphate of lime, of which it contains between 70 and 80 per cent; but of course it is devoid of nitrogen.[219] Bone-ash is best used in the dissolved form, as it possesses no characteristic action such as is possessed by bones. _Bone-char or Bone-black._ When heated in a closed retort, bones are not converted into bone-ash, but into a body called bone-char. This body is similar in composition to bone-ash, except for a certain percentage of charcoal--amounting, on an average, to 10 per cent. It contains but little nitrogen and other organic matter. Bone-black or bone-char is an article which is prepared in enormous quantities for use in sugar-refineries, where it is used in the purification of sugar. After use it may be renovated by submitting it to heat; but as this process gradually lessens the percentage of carbon it contains, after a certain period it becomes too poor in this substance for efficiently acting as a filter. When this takes place it is technically known as spent char, and is used for the manufacture of superphosphates. Spent char is a highly phosphatic substance, being very little poorer than bone-ash, and containing about 70 per cent of phosphate of lime.[220] FOOTNOTES: [216] It is only fair to Liebig to say that when he wrote these words the practically boundless supply of mineral phosphates which we now know to exist in many parts of the world was little dreamt of. [217] See Appendix, Note I., p. 371. [218] See Appendix, Note II., p. 371. [219] See Appendix, Note III., p. 372. [220] See Appendix, Note IV., p. 372. APPENDIX TO CHAPTER XI. NOTE I. (p. 364). The following analysis will serve to show the composition of bone-meal:-- Moisture 10.43 *Organic matter 32.30 Phosphate of lime 48.40 Carbonate of lime, magnesia, &c. 7.20 Insoluble siliceous matter 1.67 ------ 100.00 ------ *Containing:-- Nitrogen 3.71 Equal to ammonia 4.51 NOTE II. (p. 368). COMPOSITION OF DISSOLVED BONES. The accompanying analysis may be taken as representing the average composition of dissolved bones:--