ARTICLES 225 



be 47 cm. It should be noted that these values refer to 

 tubes without a capillary tube in the centre, such as there 

 is in the ordinary arrangement of, say, Ramsay and Shields* 

 apparatus ; with this arrangement a still wider tube would be 

 required. 



In calculating the height to which the liquid is raised, a 

 correction has to be employed for the liquid contained in the 

 meniscus above the lowest point (where the measurement is 

 made). In very narrow tubes the surface of the liquid is 

 practically hemispherical, and the volume of the liquid is then 

 the difference between the volume of a cylinder of height r 

 and that of a hemisphere of radius r {i.e., Trr^— §7rr' = ^7rr'), 

 so that the correction is made by adding one-third of the radius 

 of the tube to the observed height h. A more exact relation has 

 been worked out by Poisson for tubes of small radius, who found 



h = h^ -\- r/3 -o-i288rV/fo- 



This result has been confirmed and extended by Lord Rayleigh, 

 who gives 



2TJgpr = h -\- r/2 — o-i2SSryh + o'i2,i2r*lh'. 



Richards and Coombs measure the height h„, of the 

 meniscus and use the expression k = kg -\- n.h„, where n 

 varies from \ in tubes 2 mm. in diameter to f in tubes 30 mm. 

 across. 



The first step in setting up an apparatus for the measurement 

 of surface tension by the capillary rise method consists in the 

 preparation of a capillary tube of uniform bore. A piece of 

 glass tubing about i cm. in diameter and fairly thick walled is 

 chosen : this is heated to redness at the centre until it softens. 

 An assistant holding one end then walks rapidly away until the 

 tube is pulled out to a length of ten or twelve feet. The central 

 part of the capillary is cut into lengths of about 10 cm., and these 

 are examined for uniformity. The capillary tubes can be 

 cleaned by making a pin-hole in a piece of rubber tubing, one end 

 of which is closed by a piece of glass rod and the other is attached 

 to the water pump ; one end of the capillary is pushed into the 

 pin-hole, and the other is placed in a beaker containing the clean- 

 ing liquid, which is thus sucked through the tube, the usual 

 liquids — chromic acid, distilled water, alcohol, and ether — being 

 used. A thread of clean mercury is then drawn into the tube, 

 and its length is measured at various points in the tube by laying 

 it alongside a standard scale on a measuring microscope giving 

 good magnification. A tube should be rejected which shows a 

 variation in the length of the thread of more than i in 1,000. 



