224 WELLS'S NATUKAL PHILOSOPHY. 



If a body, at any temperature, be placed among other bodies, it will affect 

 their condition of temperature, or as we express it, thermally ; just as a candle 

 brought into a room illuminates all bodies in its presence ; with this difference, 

 however, that if the candle be extinguished, no more liglit is dift'used by it ; 

 but no body can be thermally extinguished. All bodies, however low be 

 their temperature, contain heat, and therefore radiate it. 



If a piece of ice be held before a thermometer, it will cause 

 tticrmofneter* *''® mercury in its tube to fall, and hence it has been sup- 

 sink when posed that the ice emitted rays of cold. This supposition is 

 lceT° ^^^ erroneous. The ice and the thermometer both radiate heat, 



and each absorbs more or less of what the other radiates to- 

 ward it. But the ice, being at a lower temperature than the thermometer, 

 radiates less than the thermometer, and therefore the thermometer absorbs 

 less than the ice, and consequently falls. If the thermometer placed in the 

 presence of the ice had been at a lower temperature than the ice, it would, 

 for like reasons, have risen. The ice in that case would have wanned the 

 thermometer. 



What do we ^^'^' I^'^^diations, or effects which are propagated in straight 



mean by rays lines only (such as light and radiant heat), are most conve- 

 Ueht?^* °^ niently considered by dividing them into innumerable straight 



lines, or rays; not that there are any such divisions in nature, 

 but they enable us more readily to comprehend the nature of the phenomena 

 with which these principles are concerned. 



When radiant 510. When rajs of heat radiated from one 

 ?hf sm-faTo? ^odj fall upon the surface of another body, Ihey 

 may^it^be di^ maybe disposed of in three ways : 1. They 

 posed of? jjj.-^y rebound from its surface, or be reflected ; 



2. They may be received into its surface, or be absorbed ; 



3. They may pass directly through the substance of the 

 body, or be transmitted. 



511. A ray of beat radiated from the sur- 



In what man- „ /> i i i • 'it -i 



ner is heat re- tacc 01 a Dody procccds m a straiiijht line until 



fleeted' " 



it meets a reflecting surface, from whicb it 

 rebounds in another straight line, the direction of which 

 is determined by the law that the angle of incidence is 

 equal to the angle of reflection. 



The manner in which heat is reflected is strikingly shown by taking two 

 concave mirrors, M and N, Fig. 201, of bright metal, about one foot in diameter, 

 and placing them exactly opposite to each other at a distance of about ten 

 feet. In the focus of one mirror, as at ^A , is placed a heated body, as a mass 

 of red hot iron, and in the focus of the other mirror, as at B, a small quantity 

 of gunpowder, or apiece of phosphorus. The rays of heat, radiated in diverg- 

 ing lines from the hot metal, strike upon the surface of the mirror M, and aro 



