FIRE.—PHYSICAL PROPERTIES.
Clearness, accuracy, and brevity are the essentials of good definition. That it is no easy task to combine these, every teacher realizes.
Perhaps it is near the truth to say that fire is that operation in nature which at the same time evolves heat and light. The operation is, at the present time, supposed to be a certain vibration of ethereal or more solid substances. All matter is in motion. Whence this motion was first derived no philosopher can tell, unless he goes back to that primal source of both matter and motion, which in the beginning created the heavens and the earth, and said, “Let there be light, and there was light.”
Prof. James Dwight Dana[1] declares that the first act of creative power must have been heralded throughout the universe by a flash of light. Thus the geologist unites with the scriptural narrator, in the statement that light and heat belonged to the first day of creation, although scoffers for a long time ridiculed the idea that light could exist without the sun.
All space is supposed to be filled with a substance called ether, and that it permeates even solid material. When, for any reason, the natural motion of the molecules of matter is much increased, these molecules have the power of imparting their vibration to the ether in contact with them, and that in turn may produce vibrations in other substances, and if these vibrations come in contact with the nerves of touch, there follows the sensation of warmth or heat. If the vibrations of the ether are still more rapid, when they fall upon the retina, we have the sensation of sight, and we call the agent light. Heat and light, then, are the same. In one instance the vibration is capable of affecting one set of nerves, and in the other, two sets of nerves. The heat-vibration can be discovered by the sense of touch alone, but the light-vibration may be detected both by the eye and the touch.
This variation in sensations, when produced by the same cause, may be illustrated as follows: Apply some salt to the tongue, and place some also in a wound, the two sensations are entirely unlike. Again, the vibrations of a body may be so slow that we can discover them by touch, as showing resistance, or so rapid that they are reported to the ear as a shrill sound, or they may be increased so intensely as to evolve heat, and if still more increased in rapidity, affect the eye as light. The spectrum affords us still another illustration of this truth. Pass through a prism a single ray of light, lo, it appears on the screen in all the colors of the rainbow. Nor is this all; between the bright colors, and beyond the violet and the red are invisible lines, and the various parts of the spectrum, although all are produced by the one ray, are capable of creating quite different results. If one should place a delicate thermopile below the red color, it at once reports heat, although the eye sees nothing there. The beautiful colors of the spectrum flash their light into the eye, raise the temperature of the thermometer and affect chemical transformations, while, still more wonderful, the dark lines above the violet, though unseen and not indicated by the thermopile, act upon the sensitized plate of the photographer with decided chemical force. Thus changes in vibrations as to rapidity, length and direction make changes in the resulting sensations.
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.
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.