Professor Powell on the Irifliience of' Colour on Heat. 2^9 



and that he will perceive I have no other object than the endea- 

 vour to promote the cause of philosophical truth. 

 ^ Without further preface, then, I will commence with some 

 general remarks, bearing on the nature of the whole inquiry, 

 before I proceed to consider the experimental evidence in detail. 

 After referring briefly to the labours of preceding experi- 

 menters, on the relations of substances to heat, the author re- 

 marks, that they have all stopped short as it were at the point 

 where the influence of colour comes into consideration ; and this 

 appears to have been in several cases owing to a conviction which 

 Dr Stark regards as unfounded, that the inquiry presents dif- 

 ficulties of a kind absolutely insuperable, and which are essen- 

 tially inherent in it. A quotation which he gives from Sir J. 

 Leslie tends to put these difficulties before us in the clearest 

 point of view : — 



" On the whole, it appears exceedingly doubtful if any influ- 

 ence can be justly ascribed to colour. But the question is wi- 

 capahle of' being positively determined^ since no substance can 

 be made to assume different colours without at the same time 

 changing its internal structure." Notwithstanding this explicit 

 statement, the author expresses some surprise that Sir J. Leslie 

 and Count Rumford should have stopped short in their re- 

 searches at this point, when so wide a field of unknown proper- 

 ties lay in view. 



The experiments of Sir J. Leslie have, I conceive, distinctly 

 established the general fact, that there is some pecid'iarity of tex- 

 ture^ or arrangement of the particles of the surface, which gives 

 an increased power of absorbing, and reciprocally of radiating, 

 simple heat. In my report on Radiant Heat, published in the 

 Reports of the British Association (i. 203), I have collected in 

 one point of view all the principal results. It is true that these 

 cannot be looked upon as possessing any high degree of preci- 

 sion ; chiefly from this circumstance, that the thickness, density, 

 &c. of the coating could not be in all cases precisely equalized ; 

 and thus other circumstances than the nature of the surface 

 must have afllected the results. In some cases I believe the dis- 

 tinguished author took particular pains to equalize the coatings 

 where their nature admitted of it ; as by taking precisely equal 

 quantities in the first instance, which were afterwards dissolved, 



