142 PROCEEDINGS OF THE AMERICAN ACADEMY. 



lowed. The final product was preserved in Jena glass vessels under a 

 bell -jar. 



Disodium Phosphate. One kilogram of Merck's best disodium phos- 

 phate was dissolved in hot water in a porcelain dish and hydrogen 

 sulphide passed into the solution for several hours. After standing 

 for twenty-four hours, the solution was again heated, saturated with 

 hydrogen sulphide and filtered. The filtrate was slightly green, owing 

 to the presence of iron. The solution was boiled to expel the hydro- 

 gen sulphide and a small amount of green precipitate filtered out. 

 The filtrate was still distinctly green. The sodium phosphate was 

 then crystallized fifteen times, five times in porcelain with centrifugal 

 drainage of the crystals in a large porcelain centrifugal machine, ten 

 times in platinum vessels with centrifugal drainage of the crystals in 

 platinum Gooch crucibles. The green color concentrated in the first 

 mother licpuor. 



When tested by means of the Marsh test, this material was found to 

 contain only a mere trace of arsenic, which was estimated to be 0.01 

 mg. in ten grams of the salt. This small amount could have no effect 

 on the analytical results, especially since the percentage of silver in 

 silver arsenate is nearly the same as in silver phosphate. By means of 

 the nephelometer it was proved that this material contained no chlo- 

 ride or other substances which could be precipitated by silver nitrate 

 in the presence of dilute nitric acid. 



Sodium Ammonium Hydrogen Phosphate. The best commercial 

 microcosmic salt was recrystallized four times in platinum vessels. It 

 was tested for arsenic by Marsh's method with negative results and 

 gave no opalescence visible in the nephelometer when tested with silver 

 nitrate and dilute nitric acid. 



Preparation of Trisilver Phosphate. 



Silver phosphate was prepared by mixing dilute solutions of silver 

 nitrate with solutions of sodium and ammonium phosphates. Since it 

 is not feasible to purify silver phosphate by recrystallization, the con- 

 ditions of precipitation must be so chosen that a pure product will be 

 obtained at once. 



In order to avoid inclusion and occlusion of silver nitrate, sodium 

 nitrate, sodium phosphate, or mono- or disilver phosphate, all of the 

 solutions for precipitation were made about 0.03 N. All samples after 

 precipitation were thoroughly washed and allowed to stand in water for 

 at least twenty-four hours, in order to convert occluded acid phos- 

 phates into trisilver phosphate. Qualitative tests for nitrate with 



