Only a little, just enough to hold its particles together in the form of drops. Upon what does the strength of substances depend? Their strength depends upon their tenacity. Do the different kinds of wood vary much in strength? Yes; hickory and oak are much stronger than pine or cedar. When is a knowledge of the strength of wood necessary? It is necessary to know the strength of wood used in the construction of stores, bridges, houses, etc. Which is the most tenacious of metals? Iron is the most tenacious of metals. How is this principle in iron made useful? It is useful in the construction of suspension bridges, and in very many other ways. Can iron be drawn into wire? Yes; iron can be drawn into wire. What is this property of being drawn into wire called? It is called _Ductility_. Which are the most ductile of metals? Platinum, silver, iron, copper, and gold, are the most ductile, in the order named. When is glass ductile? When glass is melted it becomes ductile, and may be drawn into very fine threads. If we take half the air out of a room, what will the remaining half do? Its particles will separate until they fill the whole room. What is said of air when its particles are thus separated? The air is in a state of _rarity_. What is said of it when the particles are pressed closer together? The air is then said to be in a state of _density_. Are the particles of water closer together than those of air? Yes; therefore water is denser than air. Upon what does the weight of a substance depend? Its weight depends upon its density. How much heavier is steel than water? Steel is seven times as heavy as water, and is, therefore, seven times as dense. How much denser is mercury than water? Mercury is thirteen times as dense as water, and is, therefore, thirteen times as heavy. Why can a bird move through the air? Because the particles of air can be separated by the bird. Why can a fish swim through the water? Because the particles of water can be separated by the fish. Why can a needle be passed through cloth? Because the fibres of the cloth can be separated by the needle. Why can a nail be driven through a board? Because the fibres of wood can be separated by the nail. What name given to this property in these substances? This property is called _Penetrability_. What is necessary in order to penetrate a substance? We must use a substance harder than the one which we wish to penetrate. Why will iron penetrate wood? Because iron is harder than wood. Why cannot our bodies penetrate the wood? Because our bodies are softer than the wood. Why can our bodies move more easily through air than through water? Because air is more easily penetrated than water. Why can a nail be driven into a pine board more easily than into an oak board? Because the fibres of pine are more easily separated than the fibres of oak; hence, pine wood is more penetrable than oak wood.