190 OUTLINES OF NATURAL PHILOSOPHY. 



c In the case of mercury, glass either repels the fluid, 

 or attracts it less than its particles attract one another. 

 Hence the convexity of the surface that terminates 

 the column, and the depression of that surface below 

 the natural level. 



d. So also, in the movement of the drop to the small 

 end of a conical capillary tube ; if the attraction of 

 a particle of the tube be resolved into two, one in 

 the direction of the axis of the tube, and the other 

 at right angles to it, the former will be directed to- 

 wards the vertex of the cone. If the force is re- 

 pulsive, the part in the direction of the axis will 

 be toward the base. On the same principle may 

 be explained the tendency of two plates to unite, 

 whether water rises or mercury sinks between 

 them. 



e. The author of this theory considers the concave 

 surface of the fluidj and the attraction which the 

 meniscus exerts, as the principal cause of the phe- 

 nomena of capillary tubes. Ibid. p. 5. We con- 

 fessj that we cannot but think it more accurate to 

 say, that the principal and primary cause of the sus- 

 pension of the water, is the attraction of the glass, 

 which sustains the meniscus, and enables it to act on 

 the water below, without being drawn out of its place. 

 It is not the concavity of its surface which makes 

 the water in the tube press less on the bottom than 

 if its surface were plain, but it is the attraction of the 

 glass, which produces, in a manner equally direct, 

 both the concavity and the diminution of the pres- 

 sure. 



281. The 



