238 Professor Powell on the Influence of Colour on Heat 



tible with a change of distance in the screen from the hot body. 

 Now, it appears to me, that if the effect arise from secondary 

 radiation, the time of transmission of the effect, until it be as 

 great as if no screen were interposed, will depend upon the 

 thickness, conducting power, and state of surface. If the thick- 

 ness be of sensible magnitude, the time of conducting through 

 will of course vary with the distance, «. e. the intensity ; but if 

 it be insensible, the time, in any case, will also be insensibly 

 small ; and hence effect will not vary with the distance. 



I have wandered far, however, fr€«n the proposed subject of 

 this communication, to which I must now return. 



Dr Stark's first set of experiments (p. 287), is designed to try 

 the influence of the colour on the absorption of heat. He em- 

 ployed equal quantities of wool, dyed of different colours, filling 

 up the interior of a glass tube, containing a thermometer, and 

 noted the rapidity of communication of heat when the tube is 

 immersed in hot water ; the greatest effect was found with the 

 darkest coloured wool. 



Here I would ask (independently of what I have just ob- 

 served relative to the change in the material employed), is there 

 not some confusion between the absorption of radiant heat (even 

 in any sense of the word), and its communication by conduction? 

 At any rate, it is the latter property which is here shewn to be 

 affected by that change in the nature of the wool (whatever it 

 may be), which is effected by the circumstance of its being dyed 

 black. I am aware that it is perhaps impossible to mark the 

 transition from one species of effect to the other, when we come 

 to insensible distances ; but nevertheless so many distinct proper- 

 ties characterize them, that we ought surely, in any investiga- 

 tion, carefully to point out whether we mean our conclusion to 

 apply to conduction or to radiation. 



The next set of experiments would seem to be regarded as 

 referring to exactly the same class of phenomena as the last ; but 

 it does in fact belong to a totally different sort. The effect 

 tried is that of the compound radiation of lights with its power 

 of exciting heat, and of simple heat together (agreeably to my 

 conclusions), upon the bulb of a thermometer, coated with dif- 

 ferently coloured pigments. The lights of course, is most ab- 

 sorbed by the dark-coloured coating, and therefore more heat 



