8 FERGUSON, Studies in Capillarity 



Or, since 2a 2 = rh very approximately, we may put the statement in 



r 



equivalent form by saying that -7 shall be small compared with unity. It 



must be very carefully remembered, therefore, that the smallness of a 

 capillary tube is not an absolute thing, but is relative to the magnitude of 

 the surface tension to be measured. 



Similarly, where r is the radius of maximum section of a drop or 



(a)'/U — -e. -& — -£_— 



Y 



Fig. 1. 



bubble, the drop is considered to be large when -^ is small compared with 



unity. If q be the vertical distance between the plane surface of the drop 



or bubble and the plane of greatest horizontal section (see Fig. 1 a), then, 



as we shall see, q is of the order of magnitude of a, and the condition 



q l ... 



implied is that -^ is small compared with unity. 



It must be clearly understood, then, that the equations developed 

 apply, and only apply, when these conditions are fulfilled ; and that the 



