Dr Stark on the Influence of' Colour on Heat. 77 



of the nature of the substance employed, has a powerful influ- 

 ence over the absorption of caloric, and agree in a striking 

 manner with the results of the experiments made by Dr Frank- 

 lin and Sir Humphry Davy, upon bodies of very different 

 qualities. 



II. — On the Radiation of Heal by differently Coloured Substances. 



It has been stated, as a general principle, that the radiating 

 powers of bodies in regard to heat bear a proportion to their 

 absorbing powers ; that is to say, that the more quickly a body 

 is heated, the more quickly does it part with its heat. On the 

 contrary, it is known, that bodies which are the most powerful re- 

 flectors of heat, are those which, when heated, retain that heat the 

 longest. This has been more particularly noticed with regard to 

 the metals, in which, by diminishing the polish of the surface, it 

 was found that their reflecting powers were much reduced, while 

 their power of radiating or giving out heat was increased. No 

 experiments have, so far as I know, been made to prove the in- 

 fluence of colour in modifying the radiation of heat, except with 

 regard to metal balls and cylinders coated with black. 



In these circumstances, it struck me that it would be import- 

 ant to ascertain whether colour, which exerts an influence so 

 powerful over the absorption of heat, might not exert an equal 

 influence over its radiation. To ascertain this point was now 

 the object of my investigation, and my anticipations as to the 

 result were fully realized, as the following experiments demon- 

 strate. If they are not so complete as could be wished, or the 

 best that could be devised, they prove sufficiently the general 

 principle, and may pave the way for more accurate investigations. 



The first experiments which I made on the radiation of ca- 

 loric were with differently coloured wools. The colours were 

 black, red, and white, of each thirty grains weight. Having 

 rolled each round the bulb of the thermometer, as formerly 

 mentioned, and placed them in the tube, it was heated in boil- 

 ing water to the temperature of about 190° Fahr. ; and when 

 the mercury in the thermometer began to descend, and had 

 fallen to 180°, it was plunged into water at 45°, and the rate of 

 cooling accurately noted. The following were the results : 



