NOTES AND ABSTRACTS. 



passes into solution when the slag is shaken with a 2 per cent, solution 

 of citric acid under conditions defined by Wagner, who first proposed 

 the test. The utility of this method of valuation has been called in 

 question in recent years, for it has been said that the phosphate which 

 remains undissolved under the prescribed conditions has considerable 

 fertiUzing value and should therefore be taken into account in the 

 valuation. The article deals with some preliminary investigations 

 which have been carried out in the laboratory of the Department 

 of Agriculture, University College, Bangor, and on a field scale at 

 various centres in North Wales, to elucidate the problem. 



It is pointed out, among other things, that the amount of free 

 lime in modern slags is much smaller than is commonly supposed. 

 Statements are frequently made that basic slag contains 15 to 

 20 per cent, of uncombined lime. This may have been approximately 

 true of the slags produced years ago, but in slags of modern origin the 

 amount of free hme present is, on an average, approximately 2 per cent., 

 and very seldom exceeds 5 per cent. This fact, however, is not of 

 such great importance from an agricultural point of view as it might 

 appear to be at first sight, for there is strong evidence to show that, 

 although the amount of caustic lime present is small, a large pro- 

 portion of the combined lime in basic slag is present in such a loose 

 form of combination that it is capable of acting as a base in the 

 presence of acids. 



It is shown that there are at least two factors which influence the 

 solubility of the phosphoric acid of slag : — 



{a) A chemical factor : the actual form of combination in which 

 the phosphoric acid exists in the slag. The phosphoric oxide occurs 

 in slag in combination with a number of other oxides, of which silica 

 is usually one. These complex compounds differ widely from one 

 another in their solubility in dilute acids, some possessing the same 

 order of solubihty as tricalcium phosphate, while others dissolve with 

 readiness. 



(b) A physical factor : the extent of surface exposed to the solvent 

 by the slag particles. The physical factor is of the greatest importance 

 in the case of those slags which contain their phosphoric acid in the 

 most soluble form, whilst in the case of slags rich in readily soluble 

 constituents the fineness of grinding may not be so important as is 

 commonly supposed. 



The results of some tests made with slags are given, and in one case, 

 although only a little over 50 per cent, of the phosphate dissolved 

 in a 2 per cent, citric acid solution when extracted by the conventional 

 Wagner method, nearly 90 per cent, had dissolved after six such 

 extractions. It is thought possible that the phosphate which dissolved 

 in the second and subsequent extractions would have been just as 

 available to the plant as that which dissolved in the first extraction, and 

 " it is conceivable that a slag which yields only a low percentage of its 

 phosphate when extracted by the Wagner method may be quite as 

 valuable to the plant as a slag yielding a high percentage." 



