198 M. Stas on the Determination of Atomic Weights. 



copper by carbonic oxide), and lastly through five U-tubes filled 

 with pumice and sulphuric acid. These U-tubes are joined by 

 tubes made of unvulcanized caoutchouc. The last U-tube is 

 provided with a metallic stopcock. Between the copper tube and 

 the five U-tubes there is a T-piece, which is provided with 

 stopcocks at its three ends, the lowest of which is connected 

 with an air-pump. To get rid of all the air from the nitrogen- 

 purifying apparatus, the gas is passed. through, the apparatus 

 exhausted, again filled with nitrogen, and this process is fre- 

 quently repeated. 



The apparatus for the analysis was now tested. It was filled 

 with nitrogen, connected with the air-pump by a tube 25-30 

 centims.in length, and i-J millim. in breadth, pumped free from 

 air, and weighed. Heated to redness in the current of nitrogen, 

 again exhausted and weighed, the weight was not found to 

 change. It did not change even on remaining five days on 

 the balance, being thus exposed to considerable variations in 

 the temperature and in the barometer. With loads of 100 

 grms. the variation did not amount to half a milligramme. 

 A system of apparatus constructed in exactly the same manner 

 served as a counterpoise. The balance had to be enclosed in a 

 double layer of white linen, on account of the delicacy of the 

 swing. The substance was now introduced into the glass bulb, 

 everything exhausted, weighed, and the decomposition set up in 

 the current of nitrogen. Two experiments were made, with 98 

 and with 159 grms. of iodate of silver. The glass bulb in a 

 dish coated with magnesia was heated with the greatest care, and 

 more strongly only towards the end of the experiment, which 

 in one case lasted six, and in the other eleven hours. The silver 

 does not then attack the glass. It is well known that by heating 

 silver in glass, yellow spots are often formed. By heating weighed 

 quantities of glass and silver in a current of dry air to the tem- 

 perature of bright redness, the weight remains unchanged. It 

 is only when the heating is continued until the glass softens that 

 there is a considerable absorption of oxygen and formation of 

 silicate of silver. The substance which had spirted on the upper 

 part of the glass bulb during the heating of the iodate of silver 

 was decomposed by some burning charcoal. At the end of the 

 experiment the parts of the glass with which the silver had been 

 in contact were coloured yellow, or brownish yellow. Direct expe- 

 riments had shown that the glass increases considerably in weight, 

 though the total weight of the apparatus remains unchanged ; 

 silicate of silver is formed and an alkaline iodide. As in the de- 

 composition of the iodate of silver all the oxygen is combined, at 

 the end of the experiment the total weight of the apparatus ought 

 not to have changed, which was found to be exactly the case. 



