MANUFACTUEE OF BASIC SLAG. 207 



brown and black needles. The order of appearance seems, therefore, 

 regulated both by the gradually decreasing temperature and by the 

 composition of the bath of slag which becomes richer in silica and 

 less rich in phosphoric acid. Cultural experiments have been made 

 with the object of examining the action of certain elements of basic 

 slag. They will be examined further on. 



Solubility of Basic Slag. — Formerly basic slag was regarded as 

 a product of no value, because there w^as no means of dissolving it, 

 owing to its high percentage of iron and lime ; neither any more did 

 any one dream of utilizing it in agriculture, because they knew 

 very well that raw phosphates jpossess no direct fertilizing value. 

 Scheibler suggested the preparation from the basic slag of a pre- 

 cipitate, German patents 24,130 and 25,020. For that purpose he 

 treated 100 parts of basic slag in powder wdth 120 to 150 parts of 

 hydrochloric acid and precipitated the phosphoric acid by milk of 

 lime. Francke recommended the decomposition of basic slag by 

 magnesium chloride, German patent 27,106, and to convert the 

 phosphoric acid into phosphate of magnesia. G. Meyer, on his part, 

 signalized the treatment of fused basic slag with its own weight of 

 acid sulphate of potash. The silicate of potash ought to give still 

 better results. The object of all these treatments w^as to facilitate 

 grinding on the one hand, and to increase the fertilizing value of 

 the basic slag, by dissolving it, on the other hand. But up till now 

 no steel-works has felt it advisable to resort to these mixtures, though 

 the manipulations w^hich they suppose in no way hinder the pro- 

 gress of the manufacture. The last-mentioned process would, how- 

 ever, appear to have the advantage of suppressing the unpleasant 

 part which the spreading of basic slag now presents. All the pro- 

 cesses w^hich have been suggested to increase the fertilizing value 

 of this manure need not be dwelt upon here, besides they have not 

 been adopted in actual practice. It w^as first of all asserted that 

 the phosphoric acid of basic slag, looking to its origin, was less 

 soluble than that under any other form, but V. Eeis and Arens, 

 amongst others, showed that phosphoric acid combined with the 

 silicate of lime was soluble in carbonated water. It is from these 

 researches that the use of basic slag in agriculture dates. The results 

 obtained by these tw^o are given in the following table (p. 209) ; the 

 figures of this table show the great solubility of basic slag in water 

 saturated with carbonic acid compared W'ith that of the other 

 phosphates. The decomposition of tetraphosphate of lime takes 

 place according to the equation — 



P.PgCa, -f- CO, = CaH,(P0J2 + 3CaH._,(CO.)2 + H.O 



There is thus formed an acid phosphate of lime such as is also 

 found in superphosphates. The solubility of basic slag in citric acid 

 and in citrate of ammonia and also in tartaric, acetic and oxalic 



