General Science. Bertha May Clark
poker, traveled along it, and warmed it. When heat flows in this way from a warm part of a body to a colder part, the process is called conduction. A flatiron is heated by conduction, the heat from the warm stove passing into the cold flatiron and gradually heating it.
In convection, air and water circulate freely, carrying heat with them; in conduction, heat flows from a warm region toward a cold region, but there is no apparent motion of any kind.
Heat travels more readily through some substances than through others. All metals conduct heat well; irons placed on the fire become heated throughout and cannot be grasped with the bare hand; iron utensils are frequently made with wooden handles, because wood is a poor conductor and does not allow heat from the iron to pass through it to the hand. For the same reason a burning match may be held without discomfort until the flame almost reaches the hand.
Stoves and radiators are made of metal, because metals conduct heat readily, and as fast as heat is generated within the stove by the burning of fuel, or introduced into the radiator by the hot water, the heat is conducted through the metal and escapes into the room.
Hot-water pipes and steam pipes are usually wrapped with a non-conducting substance, or insulator, such as asbestos, in order that the heat may not escape, but shall be retained within the pipes until it reaches the radiators within the rooms.
The invention of the "Fireless Cooker" depended in part upon the principle of non-conduction. Two vessels, one inside the other, are separated by sawdust, asbestos, or other poor conducting material (Fig. 18). Foods are heated in the usual way to the boiling point or to a high temperature, and are then placed in the inner vessel. The heat of the food cannot escape through the non-conducting material which surrounds it, and hence remains in the food and slowly cooks it.
A very interesting experiment for the testing of the efficacy of non-conductors may be easily performed. Place hot water in a metal vessel, and note by means of a thermometer the rapidity with which the water cools; then place water of the same temperature in a second metal vessel similar to the first, but surrounded by asbestos or other non-conducting material, and note the slowness with which the temperature falls.
Chemical Change, an Effect of Heat. This effect of heat has a vital influence on our lives, because the changes which take place when food is cooked are due to it. The doughy mass which goes into the oven, comes out a light spongy loaf; the small indigestible rice grain comes out the swollen, fluffy, digestible grain. Were it not for the chemical changes brought about by heat, many of our present foods would be useless to man. Hundreds of common materials like glass, rubber, iron, aluminum, etc., are manufactured by processes which involve chemical action caused by heat.
CHAPTER II
TEMPERATURE AND HEAT
14. Temperature not a Measure of the Amount of Heat Present. If two similar basins containing unequal quantities of water are placed in the sunshine on a summer day, the smaller quantity of water will become quite warm in a short period of time, while the larger quantity will become only lukewarm. Both vessels receive the same amount of heat from the sun, but in one case the heat is utilized in heating to a high temperature a small quantity of water, while in the second case the heat is utilized in warming to a lower degree a larger quantity of water. Equal amounts of heat do not necessarily produce equivalent temperatures, and equal temperatures do not necessarily indicate equal amounts of heat. It takes more heat to raise a gallon of water to the boiling point than it does to raise a pint of water to the boiling point, but a thermometer would register the same temperature in the two cases. The temperature of boiling water is 100° C. whether there is a pint of it or a gallon. Temperature is independent of the quantity of matter present; but the amount of heat contained in a substance at any temperature is not independent of quantity, being greater in the larger quantity.
15. The Unit of Heat. It is necessary to have a unit of heat just as we have a unit of length, or a unit of mass, or a unit of time. One unit of heat is called a calorie, and is the amount of heat which will change the temperature of 1 gram of water 1° C. It is the amount of heat given out by 1 gram of water when its temperature falls 1° C., or the amount of heat absorbed by 1 gram of water when its temperature rises 1° C. If 400 grams of water are heated from 0° to 5° C., the amount of heat which has entered the water is equivalent to 5 × 400 or 2000 calories; if 200 grams of water cool from 25° to 20° C., the heat given out by the water is equivalent to 5 × 200 or 1000 calories.
16. Some Substances Heat more readily than Others. If two equal quantities of water at the same temperature are exposed to the sun for the same length of time, their final temperatures will be the same. If, however, equal quantities of different substances are exposed, the temperatures resulting from the heating will not necessarily be the same. If a basin containing 1 lb. of mercury is put on the fire, side by side with a basin containing an equal quantity of water, the temperatures of the two substances will vary greatly at the end of a short time. The mercury will have a far higher temperature than the water, in spite of the fact that the amount of mercury is as great as the amount of water and that the heat received from the fire has been the same in each case. Mercury is not so difficult to heat as water; less heat being required to raise its temperature 1° than is required to raise the temperature of an equal quantity of water 1°. In fact, mercury is 30 times as easy to heat as water, and it requires only one thirtieth as much fire to heat a given quantity of mercury 1° as to heat the same quantity of water 1°.
17. Specific Heat. We know that different substances are differently affected by heat. Some substances, like water, change their temperature slowly when heated; others, like mercury, change their temperature very rapidly when heated. The number of calories needed by 1 gram of a substance in order that its temperature may be increased 1° C. is called the specific heat of a substance; or, specific heat is the number of calories given out by 1 gram of a substance when its temperature falls 1° C. For experiments on the determination of specific heat, see Laboratory Manual.
Water has the highest specific heat of any known substance except hydrogen; that is, it requires more heat to raise the temperature of water a definite number of degrees than it does to raise the temperature of an equal amount of any other substance the same number of degrees. Practically this same thing can be stated in another way: Water in cooling gives out more heat than any other substance in cooling through the same number of degrees. For this reason water is used in foot warmers and in hot-water bags. If a copper lid were used as a foot warmer, it would give the feet only.095 as much heat as an equal weight of water; a lead weight only.031 as much heat as water. Flatirons are made of iron because of the relatively high specific heat of iron. The flatiron heats slowly and cools slowly, and, because of its high specific heat, not only supplies the laundress with considerable heat, but eliminates for her the frequent changing of the flatiron.
18. Water and Weather. About four times as much heat is required to heat a given quantity of water one degree as to heat an equal quantity of earth. In summer, when the rocks and the sand along the shore are burning hot, the ocean and lakes are pleasantly cool, although the amount of heat present in the water is as great as that present in the earth. In winter, long after the rocks and sand have given out their heat and have become cold, the water continues to give out the vast store of heat accumulated during the summer. This explains why lands situated on or near large bodies of water usually have less variation in temperature than inland regions. In the summer the water cools the region; in the winter, on the contrary, the water heats the region, and hence extremes of temperature are practically unknown.
19. Sources of Heat.