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BELL SYSTEM TECHNICAL JOURNAL 



vessel; this mixture being obtained by adding liquid air to acetone 

 until its freezing point -95 C. was reached. The carbon dioxide was 

 removed similarly by means of liquid air. Readings were taken on 

 the McLeod gauge (1) with no condensing agent, this giving the 

 pressure of the total gas evolved; (2) with frozen acetone as condens- 

 ing agent, this giving the pressure of gases less the water vapor, and 

 (3) with liquid air as condensing material, corresponding to the pres- 

 sure of the permanent gases. From the several pressures thus obtained 

 the respective volumes could readily be calculated. 



For the purpose of determining the amount of gases given up by 

 the various types of glass, the glass was first cleaned with chromic 

 acid, and washed thoroughly with water. It was then placed in the 

 sample container (made of the same glass as that being tested) and 

 the pump operated for several hours in order to dry the sample thor- 

 oughly. The pump used (a mercury vapor pump in conjunction 

 with two oil pumps) was operated until the pressure within the 

 apparatus after being trapped off from the pump remained constant 

 at about 1 x 10- 6 mm. for a period of at least two hours. The glass 

 was then heated to a temperature as close to the softening range as 

 it was possible to go without causing the container to collapse; it was 

 kept at this temperature until the gas pressure became constant, 

 when the volumes of the gases were determined in the manner out- 

 lined above. The period required for completely driving off the 

 gases ranged from sixty-five to eighty hours. 



Types of Glass Used 

 The types of glass worked with, together with their chemical com- 

 positions are given in Table I. 



TABLE I. 



Chemical Analysis of the Glasses Used 



By difference. 



