-----------+--------------------------------------------------------- Higher | Lower Density, _d._ Density, D +-----+-----+-----+-----+---+-----+---+-----+-----+---+--- (Twaddell).| 14. | 13. | 12. | 11. |10.| 9. | 8.| 7. | 6. | 5.| 4. -----------+-----+-----+-----+-----+---+-----+---+-----+-----+---+--- 42 |200 |223 |250 |281·8|320|367 |425|500 |600 |740|950 40 |185 |207 |233·3|263·6|300|344·4|400|471·4|566·6|700|900 38 |171 |192 |216·6|245·5|280|322·2|375|442·8|533·3|660|850 36 |157 |177 |200 |227·3|260|300 |350|414·3|500 |620|800 34 |143 |161·5|183·3|209·1|240|277·7|325|385·7|466·6|580|750 32 |128·6|146 |166·6|191 |220|255·5|300|357·1|433·3|540|700 30 |114·3|130·6|150 |172·8|200|233·3|275|328·5|400 |500|650 28 |100 |115·3|133·3|154·6|180|211·1|250|300 |366·6|460|600 26 | 85·7|100 |116·6|136·4|160|188·8|225|271·4|333·3|420|550 24 | 71·4| 84·6|100 |118·2|140|166·6|200|243 |300 |380|500 22 | 57·1| 69·2| 83·3|100 |120|144·4|175|214·4|266·6|340|450 20 | 43 | 53·6| 66·6| 81·8|100|122·2|150|185·7|233·3|300|400 18 | 28·6| 38·4| 50 | 63·7| 80|100 |125|157 |200 |260|350 16 | 14·3| 23 | 33·3| 45·5| 60| 77·7|100|128·5|166·6|220|300 -----------+-----+-----+-----+-----+---+-----+---+-----+-----+---+--- _Lime and Limestone._--Carbonate of soda and recovered ash are converted into caustic soda by means of lime. About sixty parts of lime are necessary for the conversion of 100 parts of carbonate of soda. Large quantities of insoluble carbonate of lime are produced in this operation, and great care is necessary to prevent a loss of caustic soda which occurs if the residue is not thoroughly washed. In some cases the residual chalk is drained by vacuum filters in order to remove all traces of soluble alkali. Processes have been devised for calcining the residue so as to convert the carbonate into caustic lime to be used over again, but no economical and practical method has yet been found. The treatment of the residual chalk with sulphuric acid for the production of calcium sulphate appears feasible, but the substance obtained is very impure, and therefore has little commercial value. Limestone is required in considerable quantity for the preparation of sulphite of lime for the manufacture of wood pulp. _Recovered Ash._--The black liquor obtained during the process of the boiling of straw, esparto, and other paper-making fibres contains a large proportion of non-fibrous organic constituents derived from the fibres, the quantity of which may be gauged from the fact that these fibres generally lose 50 per cent. of their weight when being boiled. The black liquor on evaporation yields a thick resinous mass, which is converted into carbonate of soda when burnt.