The Propagation of Heat 197 



become sensible as heat, as was shown by the behaviour 

 of the thermometer in the bright can. A common 

 kitchen experience confirms our observations on this 

 head. The water in a new kettle, when placed over 

 a bright fire, takes a longer time to reach boiling-point 

 than does water in the same kettle after repeated use 

 has covered the bottom of the vessel with a layer of 

 soot. Kettles for use on kitchen ranges should not be 

 kept too scrupulously clean as regards the bottom of 

 the utensil. On the other hand, the second part of 

 our experiment suggests that the upper part of the 

 kettle should be scoured if we desire that water which 

 is being, or has been, heated in it, shall not lose the 

 heat gained more rapidly than necessary. 



131. The observations made above regarding the 

 better absorption and radiation of heat by a dull and 

 blackened surface than by a bright one are of general 

 application. Dark-coloured, rough, or dull surfaces are 

 in this respect contrasted with light, smooth, and 

 bright ones. A rough test of this may be made 

 by performing Experiment 71 again, placing two 

 thermometers at equal distances from the source of 

 heat, the bulb of one being covered with black paper 

 and that of the other with white. Using instruments 

 of similar construction, it will be found that the one 

 with the black paper round it gains heat the more 

 rapidly. Heat rays resemble light rays in being 

 reflected from bright and smooth surfaces, hence bodies 

 possessing either or both of these do not absorb heat 

 readily. 



Dark clothes are usually worn in cold weather in 

 preference to lighter garments, and there is some show 

 of reason in the choice. Any heat which reaches the 

 clothes is absorbed to a large extent, but it is an open 



