796 EXPERIMENT STATION RECORD. 



which is used m large excess when the iron content is low. In this 

 case tlie excess of sulphuric acid is saturated with iron hydrate which 

 is obtained as a by product in a subsequent stage of the ])rocess. To 

 the solution of phosjdiate of iron in sulphate of iron thus obtained 

 water is added, Avliich precipitates a considerable portion of the phos- 

 phate of iron. The solution renuiining is used for the decomposition 

 of anotlier portion of phosphate, additional sulpliuric acid being added 

 to it. Tlie phospliate of iron obtained is decomposed with potash solu- 

 tion, or if an apparatus for dialysis on a large scale is provided an 

 ammoniacal solution of the phosphate of iron may be dialyzed to the 

 point where the phosphate of ammonia disappears and the undecom- 

 posed residue of phosphate of iron is then treated with potash solution. 



The solubility of basic slag, 0. H. Rise ale ( CVtem. ¥eics, 71 (1895), 

 JHfo. 1838, pp. 82, 83). — In this paper, read before the Cleveland Insti- 

 tute of Engineers January 14, 1895, the author criticises Wagner's 

 conclusions regarding the relation between citrate solubility, silica 

 content, and fertilizing value of slags (E. S. R., 6, p. 625). He esjie- 

 cially dissents from the expression of results in terms of percentage 

 solubility. 



Investigation showed that tetrabasic phosphate of lime, whether 

 natural or artificial, was completely dissolved in Wagner's citrate solu- 

 tion, and "that increased quantities of poorer slags, containing the 

 same quantity of soluble phosphoric acid as richer varieties, and 

 which should, when treated with fixed quantities of solvent (as would 

 under actual agricultural conditions be the case), yield an etpial 

 amount [of phosphoric acid], do not, but yield nuich less." It was also 

 shown that "the alleged close relation between the silica content and 

 the percentage solubility had almost as many exceptions to the rule as 

 exami)les of it, and that there was a point beyond which the higher 

 the silica the less the solubility." 



A study of the conditions influencing solubility lead to the following 

 conclusions: 



"The percentage aolubility depends on — 



" (1) The actnal content of phosphoric acid, vnryins inversely to it. 



"(2) The neutrality or freedom from excess of lime (and possibly certain other 

 bases). 



"(3) Silica does not j>er se increase this, but only (a) when it combines with and 

 thus neutralizes excess of lime, and possibly certain other bases; (&) when as a con- 

 dition of the presence of a large quantity of silicate of lime (which silicate appears 

 not to interfere with the solubility in the quantities in which it is generally present) 

 there is only a low total percentage of phosphoric acid present. 



"(4) Silica decreases this when present in quantities greater than required in 3, 

 or sufficiently large to bring about the formation of silicates of iron or manganese. 



" (5) Excess of oxids of iron and manganese have a very small effect, almost negli- 

 gible when compared with that of lime, the actual solubility being the resultant 

 of the inert action of such of these conditions as are i)resent. 



"The absolute solubility, or actual anionnt dissolved, depends on the actual con- 

 tent of phosphoric acid, varying directly with it, and in other respects following 2, 

 3, 4, and 5." 



