[Illustration: Decoration.] [Illustration: Decoration.] CHAPTER VIII. Attraction. Cohesion and Adhesion. [Illustration: Illustrated Capital W] Why can a cup be lifted by taking hold of the handle? Because the particles of the cup do not separate from each other. What is the power which holds these particles together called? It is called _Cohesion_. In what bodies is cohesion the strongest? Cohesion is strongest in solid bodies. Is the cohesion alike in all solids? No; the cohesion in iron is much greater than the cohesion in wood. Upon what does the strength of a substance depend? The strength of a substance depends upon the amount of cohesion among its particles. What effect does heat have upon the cohesion of bodies? Heat tends to destroy the cohesion of bodies by separating their particles. How does heat affect ice? Heat destroys the cohesion between the particles of ice, and thus it is changed into water. How does heat affect water? When still more heat is applied, the cohesion in the water is destroyed, and thus it is changed into steam. Is there any cohesion in steam? No; there is not any cohesion in steam. Is there any cohesion in air? No; because the particles of air are always trying to separate from each other. Is there any cohesion in liquids? Yes; there is some cohesion in liquids, but it is only strong enough to hold them together in drops. Does the cohesion in liquids vary? Yes; the cohesion in mercury is stronger than it is in water; therefore, a drop of mercury is larger than a drop of water. How is it in chloroform? The cohesion in chloroform is weaker than it is in water; therefore, a drop of chloroform is smaller than a drop of water. How is shot made? When lead is melted and poured through a sieve at the top of a high tower, the attraction of cohesion forms it into round drops or globules; these cool, while falling, thus forming the grains of shot. How high must a shot-tower be? A shot-tower must be not less than two hundred feet high, so that the lead may harden while falling. Why does putty stick to a pane of glass? Because there is an attraction between the putty and the glass. What is this attraction called? It is called _Adhesion_. Upon what does adhesion depend? Adhesion depends upon the attraction which substances have for each other. Has water any attraction for oil? No; therefore, there is no adhesion between oil and water. Has glass any attraction for water? Yes; therefore, the water collects on the glass in drops. What other examples of adhesion may be given? Paint adheres to wood, dust to furniture, and tin and mercury to the back of a looking-glass. Will you name a few of the most adhesive substances? Glue, mortar, paste, and putty. Of what use is glue? Glue fastens pieces of wood together. Of what use is mortar?