We find air everywhere around the earth. Does the air fill every place? The air fills every place not filled by some other substance. When we say a bottle is empty, what do we mean? We mean that it does not contain anything but air. What do all vessels contain when empty? All vessels contain air when seemingly empty. Do all liquids contain air? Yes; all liquids contain air. Where is the air in a liquid? The air is among the particles which compose the liquid. Is there air in water? Yes; there is air in water. Of what use is the air in the water? It is used by the fish and other creatures that live in the water. How do the fish and other creatures use it? They breathe it into their lungs, and it supports life in them, the same as the air which we breathe supports life in us. Do solids contain air? Yes; solids contain more or less air. Where is the air in fur and in wool? It is among the hairs of the fur and the wool. Where is the air in cotton and linen goods? It is among the fibres of the cotton and the linen. Where is the air in fruit? It is among the particles composing the fruit. Where is the air in wood? It is in the pores or small spaces in the wood. Is there air in the earth? Yes; there is air among the particles of the earth. How deep in the earth does the air extend? We cannot tell; but both air and water have been found several hundred feet below the earth’s surface. Of what use is the air in the earth? It is breathed by the worms and other creatures that live in the earth. When part of the air is taken from a room, what becomes of the air left in the room? It expands so as to fill the whole room. Are the particles of air so close together after it expands as they were before? They are not so close together. When air is thus expanded, what is said of it? It is said to be _rare_. When pressure is put upon air in a vessel, what becomes of the air? It is forced or pressed into a smaller space, and, therefore, occupies less room. When pressure is put upon air, are its particles closer together? They are closer together. When air is thus compressed, what is said of it? It is said to be _dense_. Can air be much condensed by pressure? It has been condensed so much that 100 cubic feet of air was reduced to the bulk of only one cubic foot. What makes the air so dense as it is at the surface of the earth? The pressure of the air above the surface. Does the air then become less dense the higher we ascend? Yes; because there is less air above to press upon it. Does the air become more dense the deeper we go in the earth? It does; because there is more air above to press upon it. How can we understand this? In a large heap of wool, the hairs of the wool at the bottom of the heap are pressed closely together, but they are farther and farther apart the nearer they are to the top of the heap. How high does the air extend above the earth?