162 
KIRTLEY F. MATHER 
insensible between two portions of matter separated by any 
distance which we can directly measure.’’ Consequently within 
a liquid body each molecule which is more than about the twenty- 
thousandth part of a millimeter distant from the surface is 
affected equally on all sides by cohesion with other molecules 
which surround it. But the molecules in the surface film are 
on the whole attracted inward, and tension results. The amount 
of this tension depends upon the curvature of the surface, the 
composition and temperature of the liquid, and the nature of 
the surrounding medium or media. 
Capillarity'^ is ^The action by which the surface of a liquid, 
where it is in contact with a solid is elevated or depressed.” 
Because of it the surface of the water in a glass is not plane but 
is curved upward around the margins, and for the same reason 
the top of the mercury column in a thermometer is convex 
upward instead of flat. But the phenomenon is best observed 
when liquids enter tubes or spaces of small diameter or width, 
for then it may cause the liquid to move in a direction quite 
contrary to that in which it would be impelled by gravity. The 
absorption of a drop of ink by blotting paper in which the tiny 
air spaces between the paper fibers draw the fluid upward, or 
the continual supply of oil through the wick of the kerosene 
lamp, which keeps the flame steadily burning several inches 
above the oil reservoir, are familiar illustrations Capillarity 
is in part a result of surface tension but depends also upon the 
forces of adhesion, forces which act between closely adjacent 
molecules of unlike substance, those of the liquid and of the 
containing solid. 
Capillary action may best be considered as operating in tubes 
of small diameter or in fissures of minute width. Openings into 
which a liquid is drawn with a force greater than that exerted 
by gravity may be referred to as capillary openings. In such 
openings the mean height to which the fluid rises varies inversely 
as the radius of the tube or the width of the fissure. The capil- 
^ For a clear exposition of capillarity see J. C. Maxwell’s article ‘‘Capillary 
Action,” revised by Lord Raleigh, in Encyclopedia Brittanica, eleventh edi- 
tion, 1910. 
