In the ordinary so-called raw phosphates, such as bone-meal, bone-ash, coprolites, &c., the lime and phosphoric acid are combined in the form of what is known, in chemical phraseology, as tribasic phosphate of lime. That is to say, that for every equivalent of phosphoric acid there are three equivalents of lime. Now it was naturally concluded at first that the tribasic phosphate was the form in which these two substances existed in the slag. This, however, was found out not to be the case, in the following way. On allowing the slag to cool, it was found that small but perfectly defined crystals were formed. These crystals, by careful analysis, were shown, first by Hilgenstock, to consist of a form of phosphate of lime hitherto unknown, in which four equivalents of lime were combined with one equivalent of phosphoric acid, and which was therefore called "tetrabasic phosphate."
As soon as the idea of utilising the slag as a manure was suggested, various plans for extracting its phosphoric acid, and rendering it available as plant-food, were devised. These were deemed necessary, it was thought, by the very insoluble nature of the phosphates in the slag, as well as by the supposed injurious action which would be exerted on plant-life by the protoxide of iron it contained. Accordingly, a large number of patents were taken out, "covering almost every conceivable method for treating the slag, whether practicable or not. They all in the main are combinations or variations of the following processes:—
"Processes in which the slag is smelted with charcoal, to reduce phosphates to phosphides, treated with acid, and the phosphuretted hydrogen burnt to phosphoric acid; and,
"Processes in which the slag is fused with soda or potash salts,—caustic, chlorides, sulphates, carbonates,—with or without steam being forced through, to form soluble alkaline phosphates."[234]
Many of these processes were tried; but it was found by experiment that the best and most economical way was by applying the slag direct to the ground in a state of very fine powder. Experiments further showed that it had not the injurious effect on vegetation which it was feared it would have from the protoxide of iron it contained. The discovery that its phosphoric acid existed, as has been already explained, as a tetrabasic phosphate of lime, has strengthened the opinion that this is the best method of application.
A good deal has been found to depend upon the fineness of the ground slag, with the result that it is now commonly sold on a mechanical as well as a chemical analysis—i.e., the slag is guaranteed to pass through a sieve of a certain fineness.
Solubility of Slag.
Professor Wagner of Darmstadt has carried out some extremely interesting experiments on the solubility of slag. He found that very finely powdered slag was dissolved in carbonic acid water to the extent of 36 per cent, while, similarly treated, phosphorite only dissolved to the extent of 8 per cent.[235] Another very important solvent is citrate of ammonia. Reverted (or precipitated) phosphate is entirely soluble in it, and phosphate soluble in it ought to be valued as worth more than that which is not. Now, the solubility of Thomas-slag in citrate of ammonia was found by Professor Wagner to be no less than 74 per cent, while that of phosphorite only amounted to 4 per cent. These results were corroborated by Professor S. W. Johnson, who found that of the 19.87 per cent of phosphoric acid contained in a sample of basic slag, no less than 19.57 per cent was soluble in ammonium citrate, while a finely ground sample of phosphatic rock yielded, on analysis, only 1.81 per cent soluble in citrate of ammonia, of a total of 29.49 per cent phosphoric acid which it contained. Professor Fleischer has also tested the comparative solubility of basic slag and phosphorite, by boiling them in a solution of acetic acid. The former was found to have been dissolved to the extent of 19 per cent, while the latter to only 5 per cent. A highly interesting and most important experiment was performed by Mr Heinrich Albert, of Biebrich. One gramme of basic slag and 100 grammes of peat were mixed together in a litre of water, and it was found that, after standing for fourteen days, 79 per cent of the phosphoric acid contained in the slag was rendered soluble.