=How Heat and Light Travel.=—While the little motions or vibrations, of the particles, or atoms, of gas forming the flame of a candle, the Sun, and other heat and light givers could go on just the same we could not feel their heat or see their light without something or some kind of a substance which would connect them with our bodies and our eyes, like a wire connecting a push button with an electric bell. And there _is_ something which connects us with the most distant stars and that something is called the _ether_. [Illustration: FIG. 135.—RIPPLES OR WAVES ON WATER.] To show how these little movements, or vibrations set up by the atoms of a flame impress us as heat or light do at a distance we will begin with a simple experiment to show what the ether is and how it acts. If you will stand on the edge of a pool of water and throw a stone into the middle of it you will see a little ring-like ripple, or wave start out from the point where the stone struck the water; this ripple, or water wave will continue to grow larger and larger in diameter and weaker and weaker until it reaches the edge of the pond, as shown in Fig. 135, or if the pond is very large the waves will die out before they reach the edge. Again, if you toss a number of stones into the middle of the pond one after another a series of ring-like waves will follow from the center of the pond where the stones strike the water to the edge of the pond, or until they die out. In the same manner if a bell is struck by a blow of its tongue, ripples, or waves in the air will be sent out all around the bell. These waves in the air are called _sound waves_, but they are, after all, only _air waves_. When a bell is struck the rim of the bell moves forth and back, first in one direction and then in the other, as shown in the diagram, Fig. 136, and we call these little movements of the bell _vibrations_. The movements are so small that you cannot see them, but if you put your finger on the bell you can feel them. Although the vibrations of a bell are very small they are powerful enough to set up ripples, or waves in the air as shown in Fig. 137, and when these air waves or sound waves strike the drum of the ear it vibrates just like the bell and the _auditory nerve_ of the ear carries the waves on to the brain and we hear the bell ring. [Illustration: FIG. 136.—VIBRATION OF A BELL.] It must be plain now that if there was no air connecting the bell with our ears the bell might keep on ringing and yet we could not hear it. When the air is set in motion by the vibrations of a bell, or any other device for producing from 32 to 40,000 vibrations per second, we can hear it, and when the air moves as a mass, as when the wind blows, we can feel it. Air forms a layer around the Earth that is between 200 and 300 miles thick, but out in the great space beyond there is no air. Yet the space is not empty, but it is filled instead with a substance called the ether.