4 Ferguson and DOWSON, Studies in Capillarity 



C which contains a paraffin of specific gravity 0*856 at 15 . The mano- 

 meter was read by means of a cathetometer, reading to about 5-oVo tn °f an 

 inch. The capillary tube was cleaned by means of sulphuric acid and 

 bichromate, distilled water, and alcohol, and was dried by blowing 

 through it heated and filtered air. The tube was then fused on to a 

 wider tube just below the stopcock D. (It may here be recorded that 

 the whole of the apparatus was in one piece — all connections being fused 

 together, and no rubber joints admitted at any point of the apparatus. 

 After any one determination the capillary was cut off, cleaned, and re- 

 fused into position.) 



The vessel containing the liquid under examination was placed on a 

 movable table and was raised until the surface of the liquid just came 

 into contact with a pointed index E which had been previously firmly 

 fastened to the capillary at a distance h from the end of the tube. This 

 adjustment could be made with great exactness by watching the image of 

 the index point in the liquid surface. By gently raising the bottle F the 

 surface of the liquid in the tube was forced down until the lowest point 

 of the capillary surlace, as judged by the eye looking through a low power 

 microscope, was just on a level with the bottom of the capillary tube. 

 The exactness, or rather the consistency with which this adjustment may 

 be made is shown by the experimental figures given later. The presence 

 or absence of a constant error in the adjustment was roughly tested by 

 taking readings after adjusting the pressure so that (1) the meniscus just 

 disappeared within the tube, and (2) the meniscus just protruded from the 

 tube ; the mean of these pressure readings agreed quite satisfactorily with 

 the " crucial " pressure. 



The diameter of the capillary tube was determined by taking the mean 

 of 40 readings (four sets at orientations increasing by 45 °). The readings 

 were taken first on the cathetometer used for reading the pressure gauge, 

 and then on a fine dividing engine made by the Cambridge Scientific In- 

 strument Company. 1 The two determinations agreed quite satisfactorily. 



The theory of the method is developed as follows : 



With the notation of Fig. 1 the pressure at a point a in the air just 

 outside the capillary surface is given by 



Pa = P + gph + £-, 



where P is the atmospheric pressure, T the surface tension, and R the 

 radius of curvature at the vertex of the meniscus. But 



pa = pp and/p = P + gp^ 

 giving 



2T 



-R- = s{pA - ? h ) .... (n) 



T 



Now writing <z 2 = — , we know that the height to which such a liquid 



will rise in a capillary of radius r is, for a liquid of zero contact angle 



aoV-rA,(i-+-L!JL-o-i.88y . . . (iii) 



1 With cathetometer, d = 0*0663 cm. With dividing engine d = 0*06634 cm. 



