Because the heat from the cloth expands the neck of the bottle before it does the stopper, so that the stopper is loosened. Why do stove-plates often break? Because they are fitted together in the stove so that they have not room to expand without breaking. What effect has cold upon substances? Cold causes them to contract or grow smaller. Things _expand_ unequally; do they also _contract_ unequally? They do contract unequally, and in this way may be broken. Why are lamp chimneys often broken while the lamp is burning? Because the heat of the flame causes them to expand unequally. In what other way are they broken? By a draught of cold air, or a drop of cold water, touching them, thus causing them to contract unequally. Why are they sometimes broken when the lamp is not burning? Because a current of cold air from a window or a door, blows upon them. Does quicksilver expand when heated? Like other metals, quicksilver expands as it grows warmer, and contracts as it grows cooler. Is quicksilver in the form of a solid or a fluid? It is a fluid as we usually see it. For what is quicksilver used? It is used for filling the tubes of thermometers. What is a Thermometer? It is an instrument used for measuring the degree or quantity of heat in any substance. How does heat affect the quicksilver? Heat expands the quicksilver, and causes it to rise in the tube. How does cold affect the quicksilver? Cold contracts the quicksilver, and causes it to fall in the tube. Does heat expand liquids more than solids? Heat does expand liquids more than solids. Why are liquids expanded more than solids? Because the particles of which they are formed are more easily separated or pushed apart by the heat. If we continue to apply heat to a liquid, what becomes of it? The liquid is changed into a gas or vapor. What common example have we of vapor? Water, when heated, is turned into steam. Mention some gas. The _air_ we breathe is a gas. How does heat affect the air? Heat warms the air and causes it to expand. Do solids all expand equally when heated? No; some solids expand more than others; thus, zinc expands more than iron, and iron more than glass. Do liquids all expand equally when heated? No; liquids, although more sensitive to heat, do not expand as equally as solids. Do gases expand equally when heated? Yes; gases expand uniformly; thus, 491 cubic inches of any gas, if heated one degree, will become 492 cubic inches. If one cup be filled with lead, and another cup of the same size with water, which will weigh the more, the lead or the water? The lead will weigh more than the water. Which is the heavier substance, lead or water? Lead is heavier than water. Why do we say lead is heavier than water? Because a cup full of lead will weigh more than the same cup fall of water, or any bulk of lead will weigh more than an equal bulk of water. Why is cork lighter than water?