Dewars Method of producing High Vacua. 



507 



As soon as the mercury could be got to enter the capillary 

 o£ this gauge (corresponding to a pressure of about '08 mm.) 

 readings were taken every five minutes. The process of 

 filling the gauge with mercury, taking the reading, and 

 emptying it again occupied almost exactly a minute. During 

 this interval o£ time, the air contained in the gauge was cut 

 off from communication with the charcoal. 



§ 16. The results obtained with this absorber are recorded 

 in Table V. 



Table V. 



Charcoal Bulb containing 65 gms. of Charcoal. 







Initial 

 Pressure. 



Final 



Pressure. 



Values of A. 



Baro- 

 meter. 



•258 

 •363 

 •315 



MeLeod. 

 •0468 



♦0476 



•0467 



April 8th... 

 April 10th... 

 April 12th... 





763-153 mm. 

 =610 mm. 



757-380 mm. 

 =377 mm. 



766-403 mm. 

 =363 mm. 



•0109 

 •0163 

 •0214 



Charcoal bulb had been 

 strongly heated. 



P 2 5 drying-tube inserted. 

 Charcoal bulb again heated. 



The figures in the first two columns show the pressure in 

 the apparatus at the commencement and at the conclusion 

 of the experiment. It is noteworthy that the final pressure 

 attained is greater each time the charcoal is used, illustrating 

 the deterioration that takes place when the charcoal is ex- 

 posed to a damp atmosphere even though the exposure is of 

 short duration. 



§ 17. The pressures in the apparatus, as determined by the 

 height of the mercury in the barometer-tube, were plotted on 

 semi-logarithmic paper with the times as abscissas, and the 

 resulting curve was found to approximate very closely to a 

 straight line. The pressure at any instant may accordingly 

 be represented by the formula 



logp = A — Xt, 



which is of the same form as that already found (m.) 7 p 

 being now so small that it may be neglected in comparison 

 with p. The values of A from these experiments are given 

 in the Table in the last column but one. 



The pressures recorded by the MeLeod gauge were treated 

 in a similar manner, but in this case p can no longer be 



