NOTE V. (p. 396).
The following table shows the relative trade values of phosphoric acid in different manures:—
| I.—Wolff, 1893. | |
| Phosphate soluble in water (as in super) | 100 |
| Precipitated phosphate, Peruvian guano | 92 |
| Reverted phosphate, finest steamed bone-dust fish-guano, poudrette | 83 |
| Phosphatic guanos (Baker Island), wood-ashes | 75 |
| Coarser bone-dust, powdered animal charcoal, bone-ash | 67 |
| Coarse fragments of bone, powdered phosphorite and coprolite, Thomas-slag, farmyard manure | 33 |
| II.—American, 1892. | |
| Phosphate soluble in water | 100 |
| Phosphate soluble in ammonium citrate | 94 |
| Fine bone-dust, powdered fish | 94 |
| Fine medium bone | 74 |
| Medium bone | 60 |
| Coarse bone | 40 |
CHAPTER XIV.
THOMAS-PHOSPHATE OR BASIC SLAG.
In this substance we have a most important addition to our phosphatic manures. It has been in the market since 1886, and the consumption alone in Germany in 1887 amounted to nearly 300,000 tons. In this country it is only now beginning to be used to any extent.
Its Manufacture.
Thomas-slag is a bye-product obtained in the manufacture of steel by what is known as the "basic" process. In the year 1879 an improvement in the well-known "Bessemer" process was patented by Messrs Gilchrist & Thomas. It must be explained that in the manufacture of steel from pig-iron certain impurities in the raw material have to be got rid of in order to produce a good steel. Among these impurities one of the most important is phosphorus. This is owing to the fact that even a very small percentage of phosphoric acid in steel has the effect of rendering it brittle. The extraction of the phosphorus from the raw material was formerly, however, attended with very serious difficulties, and had the effect naturally of rendering steel a costly article, inasmuch as only the purer kinds of pig-iron could be used for the purpose.