3. Repeat the experiment for expansion of liquids, showing wherein the apparatus resembles the thermometer, warm the thermometer bulb and watch the column rise; cool it and note the fall. 4. Set the bulb of the chemical thermometer in boiling water. The mercury comes to rest near 212 deg. Bury the bulb in melting snow and notice that the column falls to 32 deg. Give names for these points. Explain that a degree is one of the 180 equal parts which lie between boiling point and freezing-point. Show that 32 deg. below freezing must be 0 deg., or zero. 5. The uses of thermometers for indoors and outdoors; for dairy, sick room, incubator, and soils; maximum and minimum. Dairy thermometers registering 212 deg. Fahrenheit may be obtained; they are cheaper than chemical thermometers. EXPANSION OF AIR Half fill a flask with water and invert it uncorked over water in a plate. Apply a cloth soaked in boiling water to the part that contains air. Why does the water leave the flask? Apply cold water. Why does the water return? Any ordinary bottle may be used in place of the flask, but it is more liable to crack. Make an air thermometer. See _The Ontario High School Physics_, page 223, also _Science of Common Life_, page 41. Try to graduate it from the mercurial thermometer. Have the boys make a stand for it. _Inferences._--Heated gases rise because they expand. Hot-air balloons, winds, and heating with hot-air furnaces, all depend on this principle. SOURCES OF HEAT AND LIGHT NOTES FOR A SERIES OF LESSONS 1. THE SUN.--Our dependence on it. Valuable results of its heat. Simple notions as to its size, distance, and nature. Our earth catches a very small fraction of the sun's heat; our sun is but one of millions--the fixed stars. Show the burning effect of a lens. 2. FUELS.--Wood, oil, coal, alcohol, gas, peat, straw: where obtained; special uses of each under varying conditions; need of economy. (This is closely related to geography.) 3. ELECTRICITY.--In urban schools use the electric light or some heating device for illustration. In rural schools a battery of two or three cells (see "Apparatus") will melt a fine strand drawn from a picture wire. Applications: ironing, toasting, cooking; advantages or disadvantages compared with gas or wood. 4. FRICTION.--Pupils rub hands together; rub a button on a cloth; saw a string across the edge of a board or across the hand; bore a hole through a hardwood plank, then feel the auger-bit. Applications: restoring circulation; "hot-boxes" in machinery; lubricants and their uses; lighting matches. 5. POUNDING.--Hammer a nail flat on an anvil or stone; feel it. Bullets fired against an iron or stone surface may be picked up very hot. Note sparks that can be struck from a stone; percussion caps, flint-lock muskets. 6. PRESSURE.--After using a bicycle pump for some time, feel the bottom, also the top. If possible, examine an air-compressor and find out the means used for cooling the air.