CHAMBERS'S INFORMATION FOR THE PEOPLE. 



another, rather than want of cohesion, is the grand 

 characteristic of liquids as well as gases ; it is one 

 of the causes of drops being spherical. Differences 

 of cohesion give rise to such distinctive qualities 

 of bodies as the following : 



Hardness. This quality depends not so much 

 upon the force with which the particles resist 

 separation, as upon their resisting displacement or 

 alteration of relative position. Softness implies 

 the opposite. Hardness is tested by one body 

 scratching another. It does not correspond with 

 density. Thus, glass scratches gold, and even 

 platinum. Steel has its hardness modified by tem- 

 pering. Malleability is a distinguishing quality 

 of some metals, and means the capability of being 

 extended into thin plates or leaves by hammering. 

 It depends upon the union of softness and tenacity ; 

 the particles shift their position without separating. 

 The most malleable metal is gold. Ductility is 

 the property by which a metal admits of being 

 drawn into wire. The most malleable metals are 

 not the most ductile. Iron is much more ductile 

 than tin or lead, though not so malleable. The 

 most ductile metal is platinum. Tenacity ex- 

 presses the quality by which a body resists being 

 torn asunder, and depends upon the intensity of 

 the cohesive force. It is not the opposite of brittle- 

 ness. Brittleness is associated with hardness and 

 unyieldingness, except within narrow limits. Glass 

 is brittle that is, is easily broken by bending or 

 crushing but a glass rod will sustain a great 

 weight without being torn asunder. Thus it is 

 both brittle and tenacious. The most tenacious of 

 all substances is steel. 



Adhesion, or, as it is sometimes called, hetero- 

 geneous cohesion, is the term applied to the 

 attraction which makes two different substances 

 stick to one another by their surfaces. Cohesion 

 acts between the particles of the same kind of 

 substance ; adhesion, between dissimilar kinds of 

 matter. Though the same force probably causes 

 both, yet the effects are so different that it is con- 

 venient to consider them separately, and give 

 them different names. 



Adhesion between solids. Particles of dust on 

 an upright pane of glass, chalk-marks on a wall, 

 sealing-wax on paper, cement, are all instances of 

 adhesion between substances of different kinds. 

 Adhesion between surfaces is the chief cause of 

 friction, and unctuous substances are interposed 

 to prevent it 



Adhesion of liquids to solids takes place much 

 more readily than that of solids to solids, because 

 in the case of a liquid and a solid the surfaces 

 come into more complete contact When the 

 hand or a rod of metal is dipped into water, a film 

 of the v/ater adheres to the surface, and is borne 

 up against its own weight ; nor can any force 

 shake it all off. Plunge a bit of gold, or silver, or 

 lead, into mercury, and a portion of the mercury 

 will in like manner adhere. Wherever we have 

 wetting, we have a case of adhesion of a liquid to 

 a solid. It is the cause that in pouring water over 

 the edge of a vessel, the water is apt to run down 

 the side of the vessel rather than fall perpen- 

 dicularly. 



But liquids do not always wet solids, or adhere 

 to them. A rod coated with grease, or the wing 

 of a water-fowl, remains dry when plunged in 

 water. Mercury does not adhere to a porcelain 

 cup, or to a rod of iron or platinum. The expla- 



196 



nation is simple. There is in every case an attrac- 

 tion between the solid surface and the liquid, but 

 it is opposed by the attraction of the particles of 

 the liquid for one another, and there can be actual 

 adhesion only when the former prevails over the 

 other. 



Capillary attraction is only a particular effect of 

 adhesion. A tube with a small bore, like a hair, 

 is called a capillary tube, from capilla, the Latin 

 word for a hair. If the end of such a glass tube 

 is dipped in water, the water is seen to rise in the 

 tube above the level of the rest of the surface, as 

 in fig. i. In a series of tubes of different diam- 

 eters, the liquid ascends highest in the smallest ; or 

 the heights are inversely as the diameters. Water 

 will be seen to rise in a similar way between two 

 glass plates placed as in fig. 3, with two of the 

 upright edges touching, and the other two slightly . 



Fig. i. 



Fig. 2. 



apart. The sustained film rises higher as the 

 plates approach, assuming the form of a particu- 

 lar curve. The fluid rises also slightly on the 

 outside of the tubes 

 and plates, and the 

 surface of the sus- 

 tained column within 

 the tube is seen to be 

 hollow like a cup. 



But liquids do not 

 always ascend in nar- 

 row tubes or spaces ; 

 it is only when they 

 wet the solid sub- 

 stance that they do 



Fig. 3- 



so. If a greasy glass tube is dipped in water ; or, 

 still better, if a clean glass tube is dipped in mer- 

 cury, the liquid inside, instead of rising, sinks 

 below the general level ; the surface of the column, 

 too, becomes convex instead of concave, as repre- 

 sented in fig. 2. 



The rise or the depression depends upon the 

 adjustment between the forces of adhesion and 

 cohesion, as in the case of wetting. When the 

 liquid wets the tube, the particles next its surface 

 have part of their weight taken away or supported 

 by adhesion, and thus a longer column is required 

 to balance the pressure of the rest of the fluid. In 

 cases where the cohesive attraction of the liquid 

 particles within the tube for one another is too 

 strong to permit them to adhere to its surface, 

 that cohesion tends to draw them away from it, 

 and downwards ; while the tube prevents them 

 from receiving the support they would have from 

 the liquid particles around them, if it were not 

 there. 



Capillary attraction is exemplified in many 

 familiar appearances, and plays an important part 

 in nature. If a piece of sponge or a lump of sugar 

 be placed so that its lower corner touches the 

 water, the fluid will rise up and wet the whole 

 mass. In the same manner, the wick of a lamp 

 will carry up the oil to supply the flame, though 

 the flame is several inches above the level of the 

 oil. If one end of a towel happens to be left in a 



