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.

In the above experiments it was found that the _fineness of grinding_
had a marked effect on the solubility of the slag, and that the finer it
was ground the greater was its solubility. This has been further
demonstrated in Professor Wagner's practical experiments. From these it
was found that finely ground slag has an action _four times_ as quick as
coarse slag; but that, as far as practical results were concerned, there
seemed to be a limit to the fineness to which it was advisable to grind
the slag, as slag above a certain fineness did not give better results
than a coarser slag. At any rate, he found that slag of a fineness so
great that it all passed through a gauze sieve, gave no better results
in his experiments than slag which left 17 per cent behind. We may say,
however, that the _finer the slag is ground, the greater will its
activity as a manure be_; and that a certain degree of fineness is
absolutely necessary to constitute it an active fertiliser. As
Professor Wagner's experiments are among the mos