^4 Capt. A. C. Egerton on the Vapour Pressure 



glass partition having in the centre a small aperture. 

 Mercury was placed in one compartment and kept at a 

 constant temperature, the other was kept cool ; the vapour 

 of the mercury flowed through the aperture and condensed 

 in the cool portion of the tube. 



The amount (G) which condensed was weighed after a 

 certain time (T), and the difference of pressure of the 

 mercury in the two compartments at the constant temperature 

 0° C. is given by 



,_ (w 1 + iv 2 )G\/_p 



P—P — rjf— 5 



where ici + iv 2 is the resistance offered to the passage of 

 the vapour by the hole and tube, and p is the density of the 

 vapour. 



The " resistance " of the aperture, if the walls are quite 



thin, is given by w v — , , where A is its area. While 



the " resistance " of the tube is found from the relation 



3<\Arf L o 1 



where L is the length from aperture to condensation point,. 

 o t is the circumference of the tube, and Aj the cross-sectional 

 area of a length oil of the tube. 



Knudsen measured the area of his small irregular apertures, 

 by means of a Zeiss Abbe " Zeichen apparatus " and plani- 

 meter. In order to simplify the method, tubes have been 

 constructed with accurately circular holes, the diameter of 

 which could be measured by a micrometer. This was also 

 done with a view to increasing the sensitiveness of the 

 method by increasing the number of holes drilled in the 

 plates, so that measurements of pressures of even a millionth 

 of a millimetre of mercury could be made. 



2. Experimental. 



The following forms of tube have been employed in this 

 work. 



Glass Tubes. — Apertures about 2 mm. Diameter of tube 

 2-3 cm. Length 20 cm. These were standardized by first 

 determining by their means known vapour pressures of 

 mercury at certain temperatures. 



Tubes with platinum plate. — These gave good results vvith= 



