Page:Elementary Text-book of Physics (Anthony, 1897).djvu/467

§ 371] movements that they do not vibrate in definite periods. Vibrations of all periods may exist; but if in a given case there were a tendency to one period of vibration more than to another, it is evident that the body would transfer to or receive from another, that is, it would emit or absorb, vibrations of that period more than of any other. Furthermore, a good radiator is a body so constituted as to impart to the medium around it the vibratory motion of its own molecules. But the same peculiarity of structure which fits it for communicating its own motion to the medium when its own motion is the greater, fits it also for receiving motion from the medium when its own motion is the less. Theory, therefore, leads us to the conclusion which experiment has established, that at a given temperature emissive and absorbing powers have the same ratio for all bodies.

370. Loss of Heat in Relation to Temperature.—The loss of heat by a body is the more rapid the greater the difference of temperature between it and surrounding bodies. For a small difference of temperature the loss of heat is nearly proportional to this difference. This law is known as Newton's law of cooling. For a large difference of temperature the loss of heat increases more rapidly than the difference of temperature, and depends not merely upon this difference, but upon the absolute temperature of the surrounding bodies. An extended series of experiments by Dulong and Petit led to a formula expressing the quantity of heat lost by a body in an enclosure during unit time. It is $$Q = m(1.0077)^{\theta}(1.0077^t - 1),$$ where $$\theta$$ represents the temperature of the enclosure, $$t$$ the difference of temperature between the enclosure and the radiating body, both measured in Centigrade degrees, and $$m$$ a constant depending on the substance, and the nature of its surface.

371. Kind of Radiation as Dependent upon Temperature.—When a body is heated we may feel the radiations from its surface long before those radiations render the body visible. If we continue to raise the temperature, after a time the body becomes red-hot; as the temperature rises still further it becomes yellow, and finally attains a white heat. Even this rough observation indicates that the radiations of great wave length are the principal radiations at the lower