424 PROCEEDINGS OF THE AMERICAN ACADEMY. 



In order to dry the bulb, it was exhausted five times by means of a 

 Sprengel mercury-pump, while the bath was filled with water at 70° C. 

 The air which entered was dried by passing through sulphuric acid and 

 over phosphoric oxide. The hot water of the bath was next replaced by 

 a mixture of hydrous and anhydrous sodic sulphate, which was kept at 

 its transition temperature by the passage of warm water through the 

 lead-pipe coil surrounding the bath. The temperature of the sodic sul- 

 phate bath was verified by very accurate thermometers.* 



It was found necessary to fill the bulb at the higher temperature, 

 because that mixture of rubber and paraffiue, used as a lubricant for the 

 stop-cock, which was of the proper viscosity at 32° C, became so stiff at 

 0° C. that the stop-cock could not be turned at that temperature without 

 leakage. 



The exhausted bulb was now filled with hydrogen, which had been 

 made electrolytically from hydrochloric acid with a zinc amalgam 

 anode. t The hydrogen was purified by passing through strong sodic 

 hydroxide and over phosphoric oxide. After filling the bulb, the 

 hydrogen was allowed to stream through it for half an hour, passing out 

 at the stop-cock e. The enclosing within the bulb an amount of hydrogen 

 suitable for the experiment is a somewhat delicate operation. In the 

 first place the connection between the bulb and the side tube C was 

 closed by mercury. By regulating the amount of mercury in the reser- 

 voir R, a pressure was obtained in the tube C which caused the mercury 

 in the Lord Rayleigh barometer to stand within a millimeter of the 

 upper iron point n when the lower point I was just in contact. The 

 pressure in the bulb itself was next made approximately equal to that in 

 C, as shown by simultaneous contacts at the points h and h' . This was 

 accomplished by adding hydrogen to or removing it from the bulb by 

 means of another small bulb with mercury inlet attached to the hydrogen 

 supply system. The stop-cock at the top of the bulb was then 

 closed. 



A more accurate adjustment of the pressure in the reservoir R was 

 now made, so that the contacts of the mercury with the platinum points 

 h and V were perfectly simultaneous ; the contact at the lower point I of 



* The Baudin thermometers No. 9389 and No. 9390, used for the determination 

 of the transition temperature of sodic sulphate by Richards and Cliurchili (Amer- 

 ican Journal of Science, 6, 201 (1898), also Zeitsclir. phys. Chem., 26, 690, were 

 employed for this purpose. Their relation to the international scale has since 

 been verified by Richards and R. C. Wells with the greatest care. 



t Cooke and Richards, These Proceedings, 23, 168 (1887). 



