A. L. Day and J. K. Clement — Gas Thermometer. 455 



some cases many times, and filtrates were not rejected until 

 they had been reduced to small volumes and had been proved 

 free from the elements looked for. Of course, the accuracy 

 of such work is most satisfactorily tested by synthetical 

 methods. Mylius and Fromm,* by using a preparation of 

 metallic zinc in which they could find no impurities, were able 

 to detect qualitatively as little as 0*l mg of lead, cadmium or 

 mercury, in a solution containing 40 grams of zinc, without 

 difficulty. Quantitatively, I have never found greater varia- 

 tions than -002 per cent in duplicate determinations of the 

 heavy metals in silver, copper or zinc, with a single exception 

 which was rejected, and some of the figures agree closely in 

 the ten thousands of a per cent. 



The Silver.— This metal as well as the gold, was prepared 

 by Mr. Eckfeldt at the Philadelphia Mint. A block weighing 

 about 100 grains was cut from a larger brick with a hard cold 

 chisel, and after cleaning, transferred to a large casserole of 

 Berlin porcelain and dissolved in a slight excess of nitric acid. 

 During the operation the dish was covered with a watch glass. 

 A small black residue was now filtered off on the felt of a 

 large porcelain Gooch crucible, washed and dried. The 

 asbestos of the felt was previously heated to redness. The 

 residue was then laid in a porcelain boat which was slipped into 

 a combustion tube containing copper oxide and heated in a 

 current of oxygen. The outflowing gas was passed through a 

 very dilute standard solution of barium hydroxide, l cc = 0"9T mg 

 of C0 2 , in which a decided white precipitate appeared at once. 

 The excess of baryta was then titrated with standard acid. 

 A blank determination previously made gave no precipitate 

 in the baryta water. This determination is of no importance 

 as regards the melting point of the silver, since the metal had 

 to be melted in graphite, but considering the source of the 

 silver and its unusual degree of purity, the determination may 

 be of some interest. What remained of the residue after the 

 carbon was burned, was extracted with aqua regia. The 

 solution was evaporated to dryness, taken up with hydro- 

 chloric acid and the gold precipitated by sulphur dioxide. 

 The filtrate from gold gave a slight black precipitate with 

 hydrogen sulphide. This precipitate weighed only 0"l mg after 

 it had been glowed in a small porcelain crucible, but it 

 remained black, dissolved in a few drops of aqua regia which 

 left a yellow stain when evaporated, and gave a very strong 

 rose color when dissolved in water and tested with a drop of 

 potassium iodide, — all characteristic of platinum. It was sus- 

 pected that a trace of platinum might exist in the acid used to 

 dissolve the silver, but a blank test on the same quantity of 



* Zeitschr. anorg. Chem., ix, 144, 1895. 



