10 FUNDAMENTAL PRINCIPLES 



15. Surface Films. — In a liquid mass, as has just been said, the parti- 

 cles are free to move; but if the mass is at rest, they do not do so, being 

 equally attracted by other particles all about them and in a state of 

 balance. This being true they are easily pushed aside by an object which 

 passes through the liquid. The surface of the mass, however, is formed 

 of a layer of molecules which are not in balance but which attract one 

 another and are attracted to those below them to a degree which makes 

 the penetration of this layer a matter of overcoming a certain amount 

 of resistance (Fig. 1). This layer of molecules is called a surface film. 

 ^^ It is th® presence of this film and 



r ^"-^f )*~*Cj the resistance it offers to penetration 



[j / \ which makes it possible, with suffi- 



(^ (^ cient care, to lay a dry needle upon 



the surface of a liquid and for it to 



CN /~\ remain there. The resistance which 



i^ V-^ this film offers to the penetration of 



OV-^ ^^ any object is also responsible for the 



V-^ ^-^ dimpling of such a surface when a 



C!^ V^ dry object is pressed down upon it. 



/ V_y If the object is pressed with a con- 



FiQ. 1.— Diagram illustrating the bai- stantly increasing force the dimple 



anced attraction exerted upon a molecule , i n i , • i 



in a mass of liquid (a) , and the unbalanced becomes gradually deeper until 



attraction upon a molecule at the surface finally, when the forCC beCOmCS Suffi- 

 (6). The two-pointed arrows (•^) indicate ■ , , , .1 ^i xi i • ^ 



mutual attraction between two molecules. ClCnt tO rupturC the film, the objCCt 



enters the liquid and is wet by it. 

 Then the liquid, as a result of adhesion between the particles of the object 

 and those of the liquid, rises on the surface of the object in a characteristic 

 way which is familiar to everyone — that is, unless the liquid is a very 

 heavy one, like mercury, in which case the surface film does not rise 

 but is depressed next to the surface of the object, to which, in other 

 words, the liquid does not adhere. Some animals, like the water striders, 

 move freely about on water supported by the surface film, while others, 

 like snails, cling to this film from below and move about hanging to it. 

 The surface film thus serves as a highway which may be traveled on 

 both its upper and its under surfaces. The strength of the surface film 

 of a thick liquid causes a drop of it to stand up on a dry surface and 

 assume the form of a flattened sphere, whereas the weakness of that of a 

 thin liquid results in its spreading out over a considerable area in a thin sheet. 



16. Mixtures. — Masses of different kinds may be associated in what 

 may be termed, in general, mixtures. Two solids reduced to a state of 

 fine division may be mixed; liquids also may be mixed, and gases as well; 

 and any two of the three, or all three, may be mixed. 



If a solid is mixed with a liquid and remains in masses of greater size 

 than molecules, the liquid is more or less turbid and the mixture is termed 



