[Illustration: SHOWING THE PRODUCTION OF ELECTRIC LIGHT FROM CARBON POINTS. _Ex._—The rods are first placed near together, then as the circuit is formed they are drawn apart, and the electric light is formed between them.] An instructive experiment illustrating the characteristics of different kinds of flame may be performed as follows: Place near each other a small alcohol lamp and a piece of paraffine candle; when lighted observe the two flames. The three cones in each can be easily discerned, the _candle_ burns with a much brighter light, showing it to be richer in incandescent carbon. Insert in each flame a piece of fine wire or narrow strip of glass, either of these will be much more quickly heated by the alcohol lamp, because its flame is richer in hydrogen. If a glass jar which is cold be placed over each, a film of vapor (H₂O) will gather on that covering the alcohol lamp with greater rapidity than on the other. If the jars remain over the flames until they are extinguished by the lack of oxygen, more carbonic anhydride (CO₂) will be formed from the combustion of the alcohol. PERCUSSION. When a blow is arrested by an object, the _motion_ is converted into heat. The ancient flint-lock gun and the percussion-cap fire-arm both illustrate this fact. In the former, the descending flint struck out the spark, and in the latter the cap is exploded by the arrested hammer. The stroke of a cannon ball is attended with a flash. If the world were suddenly stopped in its course, heat enough would be generated to set it on fire. Nitro-glycerine and dynamite are exploded by percussion. Familiar illustrations of this scientific truth meet us in everyday life. It has even passed into a proverb with a moral application, that “hard cracks make the sparks fly.” A novel effect of percussion may have been noticed when a fall upon the ice has resulted in a mechanical disturbance of the optic nerve which revealed whole constellations of stars never yet catalogued. FRICTION. It is a spirited sight to watch the operation of sharpening tools upon a grindstone or emery wheel run by steam. Showers of sparks are produced by the friction. We often observe the same phenomenon when the brakes are applied to rapidly revolving car wheels. Rails are heated by the friction of the passing train. You may have had the misfortune, while riding, to have one of your carriage wheels become set, caused by the box of the hub, and the axle becoming so heated by friction as to “unite” their surfaces. All machinery requires constant watching and lubrication to prevent undue friction and serious wearing. Mills have not unfrequently been set on fire by rapidly revolving belts coming in contact with the woodwork. When the whale, frantic with the pain of the harpoon, darts away with lightning speed, the sailors are compelled to dash water over the spinning wheel on which the rope is wound. In all these instances motion is transformed into heat. ELECTRICITY.