RADIATION OF HEAT. 



407 



RADIATION OF HEAT. 



WHEN any physical effect is progressively transmitted or propagated in ' 

 straight lines, especially if those lines proceed in various directions round the 

 point whence the effect originates, the phenomenon is called radiation. The 

 effect is said to be radiated, and the lines along which it is transmitted are 

 called rays. 



Several natural phenomena present examples of this, of which light is by 

 far the most remarkable. Every point of a visible object emits rays of light 

 which diverge in all possible directions from that point, and it is by these rays 

 of light that the point itself becomes visible. These rays of light, in like man- 

 ner, when they proceed from a luminous object, such as the sun, or the flame 

 of a lamp, falling on other objects, illuminate them, and making the points of 

 their surfaces become new centres of radiation, render them visible. 



The secondary rays which they thus radiate by reflection meeting the eye, 

 produce a corresponding sensation, which excites a consciousness of the pres- 

 ence of the object. Radiation is likewise a property of heat. A hot body, 

 such as a ball of iron, raised to the temperature of 400, placed in the middle 

 of a chamber, will transmit heat in every direction round it. Now this heat 

 may easily be proved not to be transmitted merely by means of the surrounding 

 air, for in this case the effect would be an upward current of hot air, which 

 would ascend by reason of its comparative lightness ; on the other hand, the 

 heat which proceeds from the ball is found to be transmitted downward, hor- 

 izontally, and obliquely, and in every possible direction. It is likewise trans- 

 mitted almost instantaneously, at least the time of its transmission is utterly in- 

 appreciable. A delicate thermometer, placed at any distance below the ball, 

 will be immediately affected by it, and the proof that this is true radiation, is 

 found in the fac that the ray may be intercepted by a screen composed of a 

 material not pervious to heat. The rays may be proved to be transmitted in 

 straight lines in exactly the same manner, and by the same reasoning, as is ap- 

 plied to rays o/ light. 



