That atoms really vibrate at the above rate per second is very certain, for their vibrations produce ether-waves the length of which may be accurately measured. When a tuning-fork vibrates 500 times a second, and the sound travels 1100 feet in the same interval, the length of each wave will be found by dividing the velocity in the air by the number of vibrations, or 1100 ÷ 500 = 2.2 feet. In like manner, when one knows the velocity and wave-length, he may compute the number of vibrations by dividing the velocity by the wave-length. Now the velocity of the waves called light is 186,000 miles a second, and a light-wave may be one forty thousandth of an inch long. The atom that produces the wave must be vibrating as many times per second as the fifth thousandth of an inch is contained in 186,000 miles. Reducing this number to inches we have 186,000 × 5280 × 12 ------------------- = 400,000,000,000,000, nearly. 1/40,000 This shows that the atoms are minute elastic bodies that change their form rapidly when struck. As rapid as the change is, yet the rate of movement is only one-fifth that of a comet when near the sun, and is therefore easily comparable with other velocities observed in masses of matter. These vibratory motions, due to the elasticity of the atoms, is what constitutes heat. THE ETHER IS ELASTIC. The elasticity of a mass of matter is its ability to recover its original form after that form has been distorted. There is implied that a stress changes its shape and dimensions, which in turn implies a limited mass and relative change of position of parts and some degree of discontinuity. From what has been said of the ether as being unlimited, continuous, and not made of atoms or molecules, it will be seen how difficult, if not impossible, it is to conceive how such a property as elasticity, as manifested in matter, can be attributed to the ether, which is incapable of deformation, either in structure or form, the latter being infinitely extended in every direction and therefore formless. Nevertheless, certain forms of motion, such as light-waves, move in it with definite velocity, quite independent of how they originate. This velocity of 186,000 miles a second so much exceeds any movement of a mass of matter that the motions can hardly be compared. Thus if 400 miles per second be the swiftest speed of any mass of matter known--that of a comet near the sun--the ether-wave moves 186,000 ÷ 400 = 465 times faster than such comet, and 900,000 times faster than sound travels in air. It is clear that if this rate of motion depends upon elasticity, the elasticity must be of an entirely different type from that belonging to matter, and cannot be defined in any such terms as are employed for matter.