Matter exists in three states: solid, liquid, and gas. It is thought that all matter may be passed through the three stages--iron being capable of being volatilized, and gases condensed to liquids and solids--the chief difference of these states being greater or less distance between the constituent atoms and molecules. In gas the particles are distant from each other, like gnats flying in the air; in liquids, distant as men passing in a busy street; in solids, as men in a congregation, so sparse that each can easily move about. The congregation can easily disperse to the rarity of those walking in the street, and the men in the street condense to the density of the congregation. So, matter can change in going from solids to liquids and gases, or _vice versa_. The behavior of atoms in the process is surpassingly interesting. Gold changes its density, and therefore its thickness, between the two dies of the mint that make it money. How do the particles behave as they snuggle up closer to each other? Take a piece of iron wire and bend it. The atoms on the inner side become nearer together, those on the outside farther apart. Twist it. The outer particles revolve on each other; those of the middle do not move. They assume and maintain their new relations. Hang a weight on a wire. It does not stretch like a rubber thread, but it stretches. Eight wires were tested as to their tensile strength. They gave an average of forty-five pounds, and an elongation averaging nineteen per cent of the total length. Then a wire of the same kind was given time to adjust itself to its new and trying circumstances. Forty pounds were hung on one day, three pounds more the next day, and so on, increasing the weights by diminishing quantities, till in sixty days it carried fifty-seven pounds. So it seems that exercise strengthened the wire nearly twenty-seven per cent. While those atoms are hustling about, lengthening the wire and getting a better grip on one another, they grow warm with the exercise. Hold a thick rubber band against your lip--suddenly stretch it. The lip easily perceives the greater heat. After a few moments let it contract. The greater coldness is equally perceptible.