THE MINERS' SAFETY-LAMP: This is a most important application of conduction. Get from the tinsmith a piece of brass gauze six inches square. Raise the wick of the spirit-lamp causing it to give a high flame and bring the gauze down upon the flame till it touches the wick. Note that the flame does not rise above the gauze. Hold a piece of paper above the gauze near the flame and note that it does not take fire. Note also that the gauze soon becomes hot. The brass wires conduct the heat of the flame rapidly away so that there is not heat enough above the gauze to cause combustion. Now roll the gauze into a hollow cylinder, pin the edges together, insert a cork at each end, and have a short candle fastened to the lower one. Try to light the candle with the lamp through the gauze. It is not easily done. The miner carries a lamp made like this, so that if he should be in the presence of the explosive gas, "fire damp", it would not explode because of the wire gauze shield. CONVECTION Water is not a conductor, how then is it heated? Drop a few pieces of solid colouring matter, (analine blue, blueing, or potassium permanganate) into a beaker of cold water. Place the beaker over a heater and observe the coloured portion rise. Wet sawdust will make a good substitute for the colouring matter. A sealing jar or even a tin cup will do instead of the beaker. The stove or a dish of hot water will take the place of the lamp. PROBLEMS 1. Using a thermometer, see whether the water at the bottom is warmer than that at the top while the beaker is being heated. 2. Heat some oil and pour it over the surface of some cold water. Lower a thermometer into this. Does the water at the bottom soon become warm? 3. If your kitchen is provided with a hot-water tank, find out what part of the tank first becomes warm after the fire is lighted. 4. In bathing, where do you find the coldest water of a pond or still river? See _Science of Common Life_, Chapter VI; also _The Ontario High School Physics_, page 280. CONVECTION IN GASES A good apparatus may be made by cutting two holes one inch in diameter in one side of a chalk box, replace the lid with a piece of glass, place a lamp chimney over each hole and a lighted candle under one of the chimneys. Hold a piece of smoking touch-paper at each chimney in turn and note direction of air current. APPLICATIONS 1. Winds are caused by the rising of air over heated areas, allowing cooler currents to take its place. (Geography) 2. Rooms are ventilated by heating some of the air more than the rest, thus producing a current. (Hygiene) Winds are nature's means of ventilating the earth. RADIATION OF HEAT This should be taken up as an introduction to dew, frost, winds, climate, etc.