EMISSION OF EADIANT HEAT. 355 



but the opposite is the case when the ice is substituted for the 

 hot metal, the thermometer bulb now radiating away more heat than 

 it receives back in return. 



On the other hand, if this experiment with ice be made in an 

 excessively cold room during a hard frost, so that the temperature 

 of the apartment is considerably below zero, the ice will behave 

 as a comparatively hot object ; a delicate thermometer placed in 

 the focus of the second mirror (not too far away) will indicate a 

 temperature below zero when nothing is placed in the focus of the 

 first mirror, and all the arrangements are allowed to stand awhile 

 to equalise the temperature ; on placing a piece of ice in position 

 at the focus of the first mirror, the thermometer will be seen 

 to rise. In carrying out this experiment a screen must be so 

 arranged as to prevent heat radiating from the warm person of the 

 observer reaching the thermometer directly ; the piece of ice should 

 be placed in position by means of a long pair of tongs. 



Expt. 381. Emission of Eadiant Heat. The emission of heat 

 in the form of rays of low refrangibility from the surface of a hot 

 body is greatly influenced by the nature of the surface. Obtain 

 half a dozen circular canisters of polished tin plate, all of the same 

 size, brighten the surface of one with oil and whiting, paint one 

 white and another black, smoke one all over with lampblack by 

 means of a large smoky flame, and cover the other two with tight- 

 fitting jackets of calico and thin black cloth respectively. On 

 filling the canisters with hot water from the same kettle, and allow- 

 ing them to stand awhile to cool, it will be found, on placing a 

 thermometer in each, that although each necessarily becomes cooled 

 by convection to about the same extent (Chapter XX.), additional 

 cooling by radiation goes on at very different rates in the different 

 cases. The brightly polished tin radiates heat least readily and 

 cools slowest, whilst the lampblacked canister cools very rapidly. 

 The can painted black will cool quicker than the one painted 

 white ; similarly the one jacketed with black cloth will cool quicker 

 than the one jacketed with white calico ; in all cases the nature of 

 the surface, and even its colour, influences the rate of radiation. 

 The difference in radiant power thus possessed by different sub- 

 stances and surfaces is one great cause why dew deposits at night 

 time more freely on some objects than on others. Those substances 

 which are the best radiators naturally cool quickest, and there- 

 fore become cooler than others which do not radiate so freely. 

 Consequently, air charged with moisture coming in contact 

 with such bodies will be sooner chilled down below the dew 

 point (Expt. 45), and will deposit moisture more copiously than 

 when in contact with substances that have not become so much 



