Nitrogen Thermometer from Zinc to Palladium. Ill 



Pure nitrogen was used throughout as the thermometric gas.* 

 The storage tank was refilled several times so that not all the 

 gas was from the same original supply ; the filling of the bulb 

 was also changed several times. The bulb was first completely 

 evacuated and heated to a high temperature, after which the 

 connections and bulb were rinsed out several times with the 

 purified gas before the final filling. 



Expansion Coefficient of the Bulb. (/3). — The substitution 

 of a new alloy in place of the platin-iridium made necessary a 

 new determination of the expansion coefficient of the bulb 

 material. The method of its determination and the comparator 

 used for the purpose were fully described in the earlier articlef 

 and do not require to be repeated here. 



Three additional precautions were taken in carrying out the 

 measurements: The bar was increased in length to 0*5 meter, 

 and in diameter to 6 mm , in order to increase the sensitiveness of 

 the determination and the uniformity of temperature along the 

 bar respectively. In this case the bar was also made at the 

 same time and from the same alloy as the bulb itself, and was 

 therefore identical with it in composition. % 



In ruling the bar, the lines were spaced 0*2 mm apart instead 

 of 0"5 mm , as in the previous investigation. This enabled a 

 greater number of observations to be made within a narrow 

 region than heretofore, and has thus made it possible for us 

 to avoid the error from parallax described in the previous 

 paper. § 



The third precaution involved a slight change in the com- 

 parator itself, and was made at the suggestion of Chappuis. 

 Our custom had been to verify the distance between the fixed 

 hairs of the microscopes before and after each heating by 

 measuring this distance in terms of a standard brass bar cali- 

 brated at the Bureau of Standards. The brass bar was then 

 replaced by the platin-iridium bar before the heating began, 

 and the length of the latter was measured in terms of the 

 initial distance between the fixed hairs, at intervals of 50° or 



* It was prepared by dropping a solution of 200 grams of sodium nitrite 

 dissolved in 250 grams of water, into a warm solution containing 350 grams 

 of ammonium sulphate and 200 of potassium chromate in 600 of water. It 

 was passed through a mixture of potassium bichromate and sulphuric acid 

 and stored over water. For use in the gas thermometer it was purified by- 

 passing through calcium chloride, hot copper gauze, potassium bichromate 

 in sulphuric acid, 2 bottles potassium pyrogallate solution, sulphuric acid, 

 calcium chloride and phosphorous pentoxide. 



•f Day and Clement, loc. cit., p. 425. 



\ The new bulb, as well as the bar, were made with the utmost care by Dr. 

 Heraeus, of Hanau, Germany, for this investigation. We have had repeated 

 occasion in the past to make public expression of our indebtedness to Dr. 

 Heraeus for his interest and assistance in this work, and it is a pleasure to 

 repeat this acknowledgment here. 



§Day and Clement, loc. cit., p. 435. 



