M. Stas on the Determination of Atomic Weights. 191 



rate experiments the condensing-tube of the still was made of 

 platinum. 



Iodaie of potassium was prepared by heating iodide of potas- 

 sium with chlorate of potassium. In order not to exceed the 

 temperature of the decomposition of iodate of potassium, a retort 

 containing chlorate of potassium alone was simultaneously heated 

 in the same sand-bath. The chloride of potassium was removed 

 by lixiviation, the iodate three times recrystallized, and each 

 time again lixiviated. The pure salt did not become yellow in 

 the air. Pure iodate of potassium can also be obtained by treating 

 iodine with caustic potash, but not by the action of chlorate of 

 potassium on iodine. 



In order easily to purify iodate of silver, it must only be pre- 

 cipitated from dilute solutions (containing 2 to 2 \ per cent, 

 of the salt) . The precipitate is shaken up with the wash-water. 

 It is washed twelve to fifteen times with cold, and then several 

 times with hot water. The precipitate is placed upon a funnel 

 which is closed by fine silver wire and some linen. By means 

 of the air-pump most of the water is removed. It is then dried 

 in a current of hot air freed from organic matter. Drying under 

 the air-pump is inadmissible ; there is always so much organic 

 matter from the grease of the plate that the silver-salt soon be- 

 comes coloured, and even blackened. The behaviour of other 

 silver-salts (for instance, the nitrate, bromate, and chlorate of 

 silver) is similar. 



Sulphurous acid was prepared by heating copper with sulphuric 

 acid which had been diluted with one-half to two-fifths of its 

 volume of water. The iodic acid was prepared by oxidizing 

 iodine with nitric acid. In this case, however, only about one- 

 fourth of the iodine used is obtained in the form of iodic acid. 

 It is evaporated to dryness, dissolved in water, again evaporated, 

 and the residue heated to 200°. But, from the operations in the 

 glass vessels, the acid always contains an admixture of soda and 

 lime. 



Hyposulphaie of silver was prepared by precipitating a manga- 

 nese salt with sulphide of barium. The filtrate is evaporated, 

 the residue dissolved in cold water, and the salt obtained recrys- 

 tallized three times. It is then precipitated by sulphuric acid, 

 and at once saturated with oxide or carbonate of silver. Neutral 

 and colourless hyposulphate of silver is thus obtained. The solu- 

 tion of hyposulphuric acid has little stability. After some time 

 it contains sulphurous acid. 



Pure sulphate of silver must be perfectly indifferent to test- 

 paper. The solution of fused nitrate of silver is poured into 

 a boiling solution of pure double sulphate of potassium. The 

 precipitate is washed with cold and then with boiling water. 



