Light-waves are always heat-waves, and heat-waves may, by increasing the rapidity of the vibrations, become light-waves. It will be observed that three of our senses are close akin. Hearing, feeling (as regards warmth) and seeing are all produced by vibrations. It is quite in accord with the doctrine of modern science to believe that the morning stars did “sing together,” for light is essentially rhythmic, and to senses adapted to the perception of their harmonies, the sunbeams would make music. The various colors of the spectrum differ solely in the wave-lengths of their vibrations. The red corresponds to low pitch in music and the violet to high pitch. As the vibrations of air striking upon the ear increase in rapidity, the sound rises in the scale. There is this difference between the ear and the eye—the former, if trained, can detect all the tones in a chord of music, while the latter, however cultivated, can not discern the varied colors blended in white light. There must be sixteen vibrations in a second to produce a continuous sound. When these vibrations reach thirty-eight thousand in a second they become inaudible. Eisenlohr[2] informs us that the red color in the spectrum has four hundred and fifty-eight trillion vibrations in a second, and extreme violet seven hundred and twenty-seven trillions. The former yields 37,640 waves in an inch, and the latter 59,750 waves in the same space. Now mark another beautiful analogy between sound and sight. In looking at the spectrum we can not discern the light or heat below the red color, because the waves are so slow. Ascending the gamut of color, the rapidity of the vibrations increases, until just _beyond_ the violet it becomes so great that the eye can detect no color. [Illustration: MECHANICAL ENERGY TRANSFORMED INTO ELECTRICITY. _Ex._—The boy on the insulated stool is repeatedly struck with some furry substance, like a tiger skin. He becomes highly electrical and capable of emitting sparks.] The same fact is discovered in the world of sound—beginning with vibrations which are too slow to be heard at all, we ascend the scale eleven octaves, when the vibrations become so rapid as to be inaudible. Complete darkness may be caused by either too slow or too rapid vibrations of light and heat, and utter silence by the same conditions in the sound waves. SOURCES OF LIGHT AND HEAT. These are five in number: The sun and stars, chemical action, percussion, friction and electricity. Stars are suns, but at a vast distance from our earth, the nearest being twenty trillions of miles away. To other systems they doubtless perform the offices of suns. Being so remote, however, although of myriad number, their influence upon our earth is hardly appreciable, and will not, therefore, be here considered. [Illustration: GEISSLER’S TUBES.[3]