5. =Spirit not an Object of Sense.=--None of the senses can perceive spirit itself, though they perceive the effects which spirit produces on material substances. If, for example, you move your arm, it is the spirit within you, acting upon the muscles through the nerves, that causes it to move; and you see here the effects produced by spirit upon matter, but you do not see the spirit itself. 6. =Effects of Matter on the Senses.=--Some forms of matter can be perceived by all the senses; others can be perceived by only a part of them; some by only one. Air you can not see, nor smell, nor taste; but you can feel it, and hear the sound of its motion. Sometimes matter affects only the sense of smell, or that with the sense of taste. Sea-air smells salt; but the salt in the air is so finely divided that we can not see it. And yet it is the salt, entering the nostrils and coming in contact with the extreme fibres of the nerve of smell, that produces the effect. So when we smell a flower, matter comes from it in particles so fine that no microscope can detect them, but they produce sensation when they strike upon the nerve. 7. =Forms of Matter.=--Matter appears in three forms: solid, liquid, and gaseous or aeriform--that is, like air. Sometimes matter is spoken of as having only two forms--solid and fluid. In this case fluids are divided into two classes, the elastic and non-elastic. The air and the various gases and vapors are the elastic fluids; while those which are called liquids are the non-elastic fluids. A foot-ball bounds because the air in it is an elastic fluid. If it were filled with a non-elastic fluid; as water, it would not bound. When water takes the form of steam it is an elastic fluid. Though it is very common to use the expression elastic fluids, the division of matter into three forms is the one usually recognized. 8. =Solids.=--In solid matter the particles can not be moved about among each other; but each particle generally retains the same position in relation to those particles which are around it--in other words, it does not change its neighborhood. This is more true of some solids than of others. It is absolutely true of such hard solids as granite and the diamond. In these the particles are always in the same relative position. But it is not so with gold or lead. By hammering these you can change greatly the relative position of their particles. India-rubber is a solid, but the relative position of its particles can be much altered in various ways.