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 |
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, Dehérain, Handtke, Petersen, Nessler, Muth, Fleischmann, Storer, and others:—
| Per cent. | ||||
| Felspar | 1.7 | |||
| Granite | 0.09 | 0.25 | 0.58 | 0.68 |
| Lava | 1.21 | 1.8 | ||
| Trachyte | 0.30 | 0.66 | ||
| Basalt | 0.50 | 1.11 | ||
| Porphyry | 0.26 | |||
| Marl | 1.45 | 2.31 | 3.8 | |
| Calcareous stones | 0.064 | 0.176 | ||
| Dolomite | 1.24 | |||
| Lias chalk | 1.39 | |||
| Gneiss | 0.18 | 0.78 | 1.51 | |
| Syenite | 0.10 | |||
| Dolerite | 0.3 | 1.1 | 1.2 | |
| Diorite | 0.5 | 0.69 | ||
CHAPTER VI.
THE POSITION OF POTASH IN AGRICULTURE.
We may, lastly, consider the position of potash in agriculture, the only ash ingredient of the plant, in addition to phosphoric acid, which it is as a rule necessary to add as a manure.