Mortar fastens together the bricks and stones used in the construction of houses and other buildings. Of what use is paste? Paste fastens pieces of paper together, and it fastens paper to the walls and ceilings of rooms. Of what use is putty? Putty fastens the glass to the wood-work of our windows. Capillary Attraction. Why does a sponge suck up water? Because the particles of the sponge have an attraction for the water. What is this attraction called? It is called _Capillary_ attraction. In what bodies do we find the most capillary attraction? In those that are of a porous or spongy nature. Why does a lump of sugar or salt, when partly dipped into water, soon become entirely wet? Because sugar and salt are porous substances, and the water rises in them by capillary attraction. Why does a heap of dry sand soon become damp when one edge touches the water? Because the water is drawn through the sand by capillary attraction. Why does water poured into the saucer of a flower-pot soon wet the earth in the flower-pot? Because the earth is porous, and the water is drawn up through it by capillary attraction. How does the oil in a lamp rise so as to be burned? The oil is drawn up through the wick by capillary attraction. Where may examples of capillary attraction be noticed? In blotting-paper when it takes up the ink, and in bread when it soaks up the juice of meats. What other examples may be given? When one end of a towel is placed in a basin of water, and the other end is left hanging outside the basin, the water will pass through the towel by capillary attraction, until the basin is entirely dry. Why is water in a glass tumbler a little elevated at the edges? Because the glass has a slight attraction for the water near it, and thus lifts it up a little. Why will water rise in a small glass tube higher than in the surrounding vessel? Because the glass has an attraction for the water, and elevates it at the edges, as is shown in Fig. 26. [Illustration: Fig. 26.] Why do salt and other substances dissolve in water? Because water overcomes the force of cohesion in the salt, and thus separates its particles. The capillary attraction between the salt and the water is greater than the cohesion between the particles of salt; hence, the salt is dissolved by the water. Gravitation. When a ball is thrown into the air, why does it fall back to the earth? Because the earth attracts the ball. Why do rain and snow fall to the earth? Because the earth attracts them. What is this kind of attraction called? It is called attraction of _Gravitation_. Does the attraction of gravitation exist in all things? Yes; every substance attracts every other substance. Upon what does the attraction of gravitation in a body depend? It depends upon the size and weight of that body. Do larger bodies attract with more force than smaller ones?