Galvanic, frictional, magnetic, thermal and animal electricity are all capable of producing heat. The first also produces an intensely brilliant light. We have long been acquainted with the “Voltaic arc”[7] of the galvanic battery, but less familiar are the magnificent manifestations of frictional electricity. Dynamo-electric machines are of comparatively recent construction, and their object is to convert mechanical energy into that of electric currents, and vice versa. A striking application of galvanic electricity is frequently seen in the discharge of gunpowder and other explosives, by making the electric current pass through a small platinum wire which is in close contact with them. Electric energy is propagated in waves, and this wire, being so small, is incapable of transmitting them all at once, so they beat upon it until their repeated blows cause it to become red hot, and the material in contact is thus ignited. Perhaps the grandest illustration of this action was seen in blowing up the rocks of Hell Gate[8] in the East River, and thus opening a safe passage for the commerce of the world. The tiny finger of a little child, the daughter of the engineer, at a given signal, pressed the key that closed the circuit, and, like Æolus,[9] when he struck the rock, set free the mighty elements of destruction. This same principle, viz.: that resisted motion becomes heat and light, is seen in both the Brush and the Edison electric lights. In the former, electric currents pass along wires to carbon points, shaped like a crayon, and covered by a film of copper, and separated by a distance of about one half inch. The air between is a non-conductor, and here the flame is formed. In the Edison light, however, the two conducting wires enter a glass globe, from which the air is excluded. Here they are connected with a spiral wire about as large as a knitting needle, and three-quarters of an inch in length. When the electricity is turned on, this spiral glows with an intensely brilliant white light. [Illustration: SOLIDS DIFFER AS TO CONDUCTING POWER. _Ex._—If we hold a pipe stem or rod of glass in one hand and a copper wire in the other, and apply the ends of these to a flame, the wire will convey the sensation much more quickly to the hand than the other. This shows that solids differ as to conducting power.] A marvelous illustration of the relation between electric and sound vibrations is found in the telephone and microphone. The former is becoming a household necessity; the latter, though not so well known, is not less wonderful. It brings to our ear the tick of a watch miles away, and through it the walking of a fly sounds like the tramp of a horse. DISTRIBUTION OF HEAT.