74 MOLECULAR WEIGHT OF ARGENTIC SULPHATE, ETC. 



The method of preparing the materials, the shape and nature of the 

 apparatus, and many of the precautions and details of manipulation were 

 adopted only after numerous preliminary experiments. A detailed account 

 of this tentative work is, however, unnecessary, since the important results 

 are implied in the description of the method which was finally adopted. 



THE PREPARATION OF PURE MATERIALS. 

 SULPHURIC ACID. 



The best commercial "chemically pure'' acid was twice fractionally dis- 

 tilled, using a quartz condenser and a platinum dish as receiver. Only 

 the middle fractions were used; 17 grams left no visible or weighable 

 residue on evaporation. 



ARGENTIC SULPHATE. 



Pure argentic chloride residues from previous atomic weight investiga- 

 tions in this laboratory were reduced with invert sugar and sodic hydrate. 

 The reduced silver was thoroughly washed, and dissolved in nitric acid ; 

 and the nitrate was diluted and filtered. It was evaporated on the steam- 

 bath until saturated, and crystallized by adding an equal volume of con- 

 centrated nitric acid and cooling. The crystals were drained centrifu- 

 gally as usual. 1 The crystallization from pure concentrated nitric acid 

 and centrifugal draining was repeated five times, using Jena-glass vessels. 

 It was finally recrystallized once more, using a platinum dish and redis- 

 tilled nitric acid. This silver nitrate was dissolved in a small amount of 

 water in a platinum dish, and an excess of the purest sulphuric acid, diluted 

 with an equal volume of pure water, was poured into it. The precipitated 

 silver sulphate was drained in the centrifugal machine. It was then dis- 

 solved in hot concentrated sulphuric acid in a platinum dish, and the solu- 

 tion was boiled for several minutes to expel nitric acid. On cooling, the 

 acid sulphate crystallized in large crystals. This acid sulphate was very 

 thoroughly whirled, placed in a platinum dish, and treated with purest 

 water. Heat was evolved and the normal sulphate crystallized out as a 

 fine powder. This powder was washed by decantation with the purest 

 water until the wash-waters were no longer acid. 2 All the water used in 

 this work was purified in the manner described in previous communica- 

 tions. During this washing the action of light on the wet silver sulphate 

 produces a slight violet color. This, however, does no harm, as any slight 

 decomposition is remedied during the subsequent fusion. It was then 



iRichards, J. Amer. Chem. Soc., 27, 104 (1905). 



2 Unless this precaution is taken the silver sulphate can not be dried thoroughly in 

 the air-bath; in which case it would be impossible to clean properly the receiving- 

 end of the tube into which it was afterwards introduced. (See p. 70.) 



