40 



PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



The importance of these facts will be seen in the explanation of 

 capillary phenomena. 



2. Adhesion is the molecular attraction exerted between tlie surfaces 

 of bodies in contact ; it may be manifested between solids (as between 

 two freshly-cut surfaces of a leaden bullet, or two pieces of plate-glass),, 

 between solids and liquids (as when a drop remains clinging to a glass 

 rod which has been dipped in water), and between solids and gases (as 

 shown by the bubbles of air which adhere to a glass or metal plate when 

 immersed in water). 



The adhesion of liquids to solids, which alone of the above will 

 receive attention at present, is greater than the cohesion of liquids, as 

 already mentioned. Thus, when a drop of water is placed on a glass 

 surface it does not assume the spheroidal form, but becomes flattened 

 out, showing that the adhesion of the water to the glass is greater than 

 the cohesion of the water. If, however, the glass plate be greasy, then 

 the drop of water will exert no adhesion to the glass and will remain 

 spheroidal. This adhesion of liquids to solids is not universal, but 



Fig. 27. Fig. 28. Fig. 29. Fig. 30. 



Capillary Phenomena. (Gemot.) 



depends on the nature of both the solid and the liquid. Certain liquids 

 are capable of adhering to, or wetting, certain solids and not others ; and 

 a solid to which one liquid will adhere will be inert, or even repulsive to 

 another. These facts also are of importance in the explanation of 

 capillarity. 



3. Capillarity. — When a solid body is placed in contact with a liquid 

 the phenomena (attraction or repulsion) which result are termed capillary 

 phenomena from the fact that they are best seen when capillary tubes 

 {capillus, a hair) are immersed in liquids. 



As already stated, water is capable of adhering to glass. When a 

 glass rod is dipped into water, the water is raised up against the sides 

 of the rod to form a concave surface above the level of the water, as if 

 no longer subject to the laws of gravity (Fig. 27). If, on the other 

 hand, a glass rod be dipped into mercury, which does not adhere to 

 it, the mercury is depressed around the rod, forming a convex surface 

 below the level of the surrounding fluid (Fig. 28). If glass tubes 

 with narrow bore are immersed in water and mercury, in the former 



