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.

[Illustration:  FIG. 18.—­A fireless cooker.]

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 deg.  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.