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." _Processes for preparing Slag._ 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:-- "1. _Preliminary preparation of the Slag._ (_a_) By treating molten, or otherwise, with superheated steam, or cooling when hot with water, to reduce it to small pieces or to a fragile state. (_b_) Grinding. (_c_) Treating with water to wash out free lime, or with sugar solution. (_d_) Roasting in the air, or with some oxidising agent. "2. _Solution of the Slag._ (_a_) _Completely_ in weak or strong acids (hydrochloric, sulphuric, &c.) (_b_) _Partially_, so as to dissolve the phosphates and silicates of lime, and leave most of the iron and manganese oxides. "3. _Precipitation_ of the phosphoric acid, with lime or iron salts: or, "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]