Professor Powell aii the Injluence of Colour on Heat. 241 



the temperature of the earth. It is a badly conducting^ substance 

 by virtue of its peculiar texture of flakes and spiculae ; and this 

 being accordant with a known general law of the effect due to 

 fiuch texture, I cannot conceive we are reasoning philosophically^ 

 in having recourse to so remote a property as its white colour^ 

 to account for the effect. 



Similar remarks appear to me to apply with equal force to 

 the experiments on the deposition of dew ; and I must confess 

 myself entirely disposed to concur in the truly philosophical he- 

 sitation displayed by Dr Wells in prosecuting such an inquiry, 

 notwithstanding the author's implied censure of it, and to regard 

 the objection cited in the words of Sir J. Leslie as almost de- 

 cisive against the validity of any conclusion, until we shall have 

 a much closer insight into the actual structure and intimate na- 

 ture of bodies than we at present possess. " A black body al- 

 most always differs from a white, in one or more chemical pro- 

 perties, and this difference may alone be sufficient to occasion a 

 diversity in their powers of radiating heat." (P. 300.) 



I have here restricted my observations to that portion of Dr 

 Stark's inquiry which relates to heat. The other part of it, in 

 which he endeavours to establish similar conclusions with respect 

 to odours, miasma, &c. refers to subjects in which I cannot pre- 

 tend to be so conversant. But I cannot help thinking that 

 many of the same cautions which I have ventured to suggest, 

 as far as they regard the philosophical character of the reason- 

 ing, might be found not less applicable in these cases also. The 

 same want of due distinction between the different sorts of heat- 

 ing effect runs through Dr Stark's historical Sketch in the Edin- 

 burgh New Philosophical Journal, (No. xxxiii, p. 65). The 

 earlier experiments to which he refers, of Des Cartes, Boyle, &c. 

 all refer not to heat in general, but to the sun's rays. Those of 

 Hooke and Franklin specially establish the analogy between the 

 solar rays and those from jlame. Those of Bishop Watson do 

 not, I would submit, refer to " the effect of a coating of black 

 in raising the temperature of substances ;" but to its effect in in- 

 creasing that particular development of heat which takes place 

 when the sun's light is absorbed, and' which is necessarily great- 

 est in dark coloured surfaces. Count Rumford's experiments 

 refer to two totally distinct inquiries : one the conducting^ the 



