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



the present series of observations was not extended beyond 

 1200°. Observations above this temperature will therefore be 

 made the subject of a subsequent paper in which a bulb con- 

 taining no ridium will be substituted for the one described 

 here. 



Up to 1200° our precautions were sufficient to prevent 

 serious contamination of the elements and the error due to 

 such contamination as was unavoidable has been eliminated 

 by frequent calibrations of the three elements throughout 

 the observations, either by comparison with standard elements 

 known to be free from iridium contamination, or with melting 

 point determinations of standard metals. Toward the close of 

 the series, in order to establish absolute proof that the readings 

 w r ere not encumbered with systematic errors, however small, 

 from this cause, an independent observation was made in the 

 following way : The element at the middle of the bulb was 

 replaced by a freshly calibrated new element known to be in 

 perfect condition. After an ice-point determination and with 

 all the precautions above described, the furnace was heated 

 directly and as rapidly as possible to 1200°, where a single 

 observation was made and the furnace immediately cooled down 

 again. The new element was then removed from the furnace 

 and recalibrated in order to establish beyond question the fact 

 that it had suffered no contamination whatever during the short 

 run. This independent determination, in which it was defi- 

 nitely proved that iridium contamination played no part, served 

 to establish the absolute correctness of the high temperature 

 observations in so far as the error from this most, persistent 

 source was concerned. 



The Gas. — Nitrogen was prepared by dropping a solution 

 of 200 grams of sodium nitrite dissolved in 250 grams of water 

 into a warm solution containing 300 grams of ammonium sul- 

 phate and 200 of potassium chromate in 600 grams of water. 

 The gas was then passed through a mixture of potassium bichro- 

 mate and sulphuric acid and stored over water. Before intro- 

 ducing this nitrogen into the gas thermometer, it was 

 purified by passing it in succession through calcium chloride, 

 hot copper gauze, potassium bichromate in sulphuric acid, 

 two bottles containing potassium pyrogallate solution, sulphuric 

 acid, calcium chloride and phosphorus pentoxicle. 



No reason has yet arisen in any of the experiments with 

 nitrogen for suspecting limitations of any kind due to the gas. 

 It has shown no tendency to react with the platinum bulb or 

 to pass through its wall or to dissociate at any temperature to 

 which it has yet been carried in gas thermometry. Further- 

 more Buckingham * has shown by ingenious methods of com- 



* E. Buckingham, loc. cit. 



