These heat waves are ether waves, differing from light only in having greater wave length. They travel at the speed of light, they travel better without air than with air. They come from the sun and all other light-giving bodies. Indeed, an ordinary incandescent electric lamp gives out about twenty-four times as much energy in heat as in light. Heat waves are being thrown off from all bodies which are around us. The steam radiators are placed in this room for the express purpose of sending out heat waves through the ether in this room. This is the chief method of distributing heat, and it is hindered rather than helped by the presence of the air. The walls, ceiling, floor, furniture, people--everything here is sending out heat waves. The fourth kinds of waves, which go out from the spark gap of our coil, are also waves in the ether. They are still longer than heat or light. We have ears for sound, eyes for light, and temperature sensation for heat, but as yet we have not evolved a delicate sense organ for detecting electric waves. At least few of us claim to have such a sense. I will, however, undertake to make you feel electricity. I then adjusted the coil so that each boy might take a mild electric shock from it by touching the two knobs. That is by placing himself in the spark gap. They agreed that although they could not hear, see, taste, or smell electricity they were a little more familiar with it now, having felt it. Sound waves in air, as given out by the piano, vary in length from, say, four inches to forty feet, those having the shorter wave length being the higher pitched tones. Light waves in the ether, as given out by the sun, vary in length from, say, 1/60000 to 1/80000 of an inch, those having the shorter wave length being the violet-coloured light, which may be seen in the rainbow, and those having the longer wave length being the red-coloured light of the rainbow or the sunset. Heat waves, which are also waves in the ether, vary in length from above 1/80000 to, say, 1/5000 of an inch. Roentgen or X waves are ether waves, shorter than light; while Hertzian, or wireless telegraph waves are very long ether waves, varying from a few feet to many rods in length. Those used by Marconi in sending despatches across the Atlantic Ocean are as long as 1000 feet, four or five of them cover a mile, and 12,000 of them cover the whole distance from Cape Cod to Poldhu. Electric waves are easily broken up into the shorter heat waves, or the still shorter light waves. On the other hand Roentgen waves are readily transformed into the longer light waves, and are thus brought within our powers of vision. Sound waves of various lengths (of high and low pitch) all travel at the same speed (one mile in five seconds), else how would the piccolo and the bass horn of the distant band sound together. So ether waves of various lengths (light, heat, electricity, etc.) all travel at the same speed, _i. e._, 186,000 miles per second.