42 



PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



solid and the line of gravity, and the surface of the liquid being at right 

 angles to that resultant will be concave (Fig. 35). 



Third.— It the attraction of the solid for the liquid decreases, or 

 the cohesive attraction of the liquid increases, the resultant will fall to 

 the other side of the line of gravity, or between the line of cohesive 

 force of the liquid and the line of gravity, and the surface of the liquid, 

 being perpendicular to that resultant, will be convex (Fig. 36). 



Fig. 35. 



Fig. 36. 



II 



Fig. 34. 



Diagrams illustrating Cause op Curvature of Liquid Surfaces in 

 Contact with Solids. ((Jaitot.) 



The molecule m is noted on by gravity, in the vertical line m P : is attracted by the plate n. in the line 

 rx m and by the liquid F. in the line m F. The direction of the resultant m R will depend upon the relative 

 intensities of these forces. If u in and m F balance, the resultant is vertical, m R (Fig. Si), and the surface 

 is horizontal If n m increases, or m F decreases, the resultant R is within the angle v m P, and the surface 

 is concave (Fi" 35) If m F increases, or n m decreases, the resultant R is within the angle PmF, and the 

 surface is conrex, for the surface of a liquid is always perpendicular to the resultant of forces acting on 

 its molecules (Fig. 36). 



The ascent or descent of liquid within a capillary tube is dependent 

 on the manner in which the curvature of the surface modifies the prin- 

 ciples of hydrostatic equilibrium. 



When a tube of large calibre is immersed in a vessel containing 

 liquid the conditions of equilibrium are the same as in two communicating 

 vessels containing the same fluid. Equilibrium is only possible when the 



surface of the liquid in both vessels is 

 on the same horizontal plane. For, take 

 any molecule in the plane MX (Fig. 37). 

 It will be subjected to a downward pres- 

 sure equal- to the weight of a column of 

 the same fluid, the height of which is equal 

 to the distance of that molecule from the 

 surface of the fluid within the tube. It 

 will also be subjected to an upward pres- 

 sure which is equal to the weight of a, 

 column of liquid whose height is equal to 

 the distance of that plane from the surface 

 of the liquid without the tube. These weights are, however, equal. 

 Therefore every molecule in the plane MN will be subjected to equal and 

 contrary pressures, and will consequently be in equilibrium. 



Suppose, however, the tube have a diameter less than one millimeter. 

 The concave surfaces produced by the adhesion of the fluid to the 



Fig. 37.— Diagram illustrating 

 Liquid Pressures. 



