As a ray of light continues a ray of light so long as it exists in free ether, so all kinds of radiations, of whatever wave-length, continue identical until they fall upon some mechanical structure called matter. Translatory motion continues translatory, rotary continues rotary, and vibratory continues to be vibratory, and no transforming change can take place in the absence of matter. The ether is helpless. 13. MATTER IS ELASTIC. It is commonly stated that certain substances, like putty and dough, are inelastic, while some other substances, like glass, steel, and wood, are elastic. This quality of elasticity, as manifested in such different degrees, depends upon molecular combinations; some of which, as in glass and steel, are favourable for exhibiting it, while others mask it, for the ultimate atoms of all kinds are certainly highly elastic. The measure of elasticity in a mass of matter is the velocity with which a wave-motion will be transmitted through it. Thus the elasticity of the air determines the velocity of sound in it. If the air be heated, the elasticity is increased and the sound moves faster. The rates of such sound-conduction range from a few feet in a second to about 16,000, five times swifter than a cannon ball. In such elastic bodies as vibrate to and fro like the prongs of a tuning-fork, or give sounds of a definite pitch, the rate of vibration is determined by the size and shape of the body as well as by their elementary composition. The smaller a body is, the higher its vibratory rate, if it be made of the same material and the form remains the same. Thus a tuning-fork, that may be carried in the waistcoat-pocket, may vibrate 500 times a second. If it were only the fifty-millionth of an inch in size, but of the same material and form, it would vibrate 30,000,000000 times a second; and if it were made of ether, instead of steel, it would vibrate as many times faster as the velocity of waves in the ether is greater than it is in steel, and would be as many as 400,000000,000000 times per second. The amount of displacement, or the amplitude of vibration, with the pocket-fork might be no more than the hundredth of an inch, and this rate measured as translation velocity would be but five inches per second. If the fork were of atomic magnitude, and should swing its sides one half the diameter of the atom, or say the hundred-millionth of an inch, the translational velocity would be equivalent to about eighty miles a second, or a hundred and fifty times the velocity of a cannon ball, which may be reckoned at about 3000 feet.