and Nitrogen Vapour at the Boiling-point of Oxygen. 361 



manently on one scale of the balance during all the weighings, while 

 the A flask was weighed, either exhausted or filled with oxygen (or 

 nitrogen), under various circumstances according as the experiments 

 required. 



As the flask cooled in the liquid oxygen or air had to stand an 

 internal pressure of from three to four atmospheres, it was considered 

 expedient to select a spherical vessel of about 300 c.c. capacity, to 

 which was sealed a narrow tube having a very carefully ground stop- 

 cock at its end. Preliminary experiments were made to determine the 

 change of volume of the flask when subjected to internal pressure. The 

 flask, filled with air at five atmospheres pressure, was left for 24 hours 

 with the stopcock closed, without showing any leakage. To determine 

 the eftect of pressure on the capacity of the flask, it was filled with 

 water under one atmosphere pressure, and again with water under 

 three to four atmospheres pressure (which included the range of the 

 observations) and the weights noted. From these the coefficient of 

 expansion of the flask was found to be 0*000306 per atmosphere excess 

 of internal over external pressure. The temperature coefficient of 

 expansion of glass (cubical) was taken to be - 000025, which for a 

 variation of temperature of some 200 altered the capacity of the flask 

 by about 1*5 c.c. The content of the flask up to a fixed mark on the 

 neck below the stopcock was determined to be 315*973 c.c. at 17 C. ; 

 the content between the mark and the stopcock was determined both 

 by measurement and by the weight of mercury it contained, and was 

 found to be 0*127 c.c. 



Before each experiment the A flask filled with the gas under observa- 

 tion was exhausted to a pressure of from 2 to 4 mm. of mercury (which 

 was afterwards involved as a correction in reducing the observations) 

 and weighed, the weight a which had to be added to its scale pan to 

 balance the B flask being noted. The A flask was then filled with 

 carefully purified oxygen (or nitrogen), and the temperature of the 

 flask and contents (still in communication with the gas reservoir) was 

 lowered by immersing it up to the mark in liquid oxygen (or air) until 

 the gas ceased passing into the flask, and the pressure was finally 

 equalised to that of the atmosphere. This is really the most important 

 part of the manipulation, as the accuracy mainly depends on giving 

 sufficient time to the cooling, while at the same time taking care to 

 avoid any excess of pressure that would necessarily lead to liquid 

 condensation on the glass surface. During the rapid inrush of gas, it 

 is advisable to keep the pressure well below that of the barometer at 

 the time, and to finally adjust the pressure at the end of the absorption. 

 The vessel containing the liquid oxygen had a cardboard cover which 

 crossed the neck of the flask at the mark, thereby preventing the 

 cooled vapour of the oxygen from freezing the stopcock. The remain- 

 ing portion of the tube between the mark and the stopcock, namely, 



VOL LXix. 2 C 



