Fifty cubic centimeters of the filtrate, representing one gram of the original substance, are placed in an erlenmeyer. In the case of double superphosphates which often contain large quantities of pyrophosphates, twenty-five cubic centimeters of the filtrate just obtained, equivalent to five grams of the substance, are diluted with seventy-five cubic centimeters of water, ten cubic centimeters of nitric acid of 1.42 specific gravity added, and heated on a sand-bath to convert the pyro into orthophosphates. The heating should be continued until the liquid is reduced to its original volume of twenty-five cubic centimeters. The strongly acid liquid is saturated with ammonia and with the addition of a drop of rosolic acid as an indicator, again acidified with nitric acid, and treated as with superphosphates. =79. Dutch Method for Basic Slag.=—Heat ten grams of the sample with fifty cubic centimeters of sulfuric acid (1.84 specific gravity) till white vapors are evolved, shaking or stirring constantly. After cooling make the fluid up to 500 cubic centimeters with water, taking no account of the undissolved substance. Filter, and to fifty cubic centimeters of the filtrate add 100 cubic centimeters of the ammoniacal citrate solution, and after cooling, twenty-five cubic centimeters of magnesia mixture. Stir or shake for a sufficient time. After the lapse of two hours the precipitate is to be separated by filtration and treated in the usual manner. =80. Estimation of Citrate-Soluble Phosphoric Acid in Basic Slag.=—Experience has shown that the manurial value of basic slags does not depend alone on their content of phosphoric acid. Slags may contain tri- as well as tetracalcium phosphate, and even this latter salt may exist in states of differing availability. In determining the availability of basic slag for manurial purposes its solubility in ammonium citrate is considered the best standard. But this solubility will evidently be influenced by the basicity of the sample, or in other words, by the quantity of lime present. A slag rich in calcium oxid would deport itself differently with a given ammonium citrate solution from one in which the lime had been chiefly converted into carbonate. If possible, therefore, all samples should be reduced to the same state of basicity before the action of any given solvent is determined.