Professor Powell on the Influence qf Colour on Heat. 237 



niometer remained in contact long enough to acquire the tempe- 

 rature completely. 



The following is a view of the results :— 



Iron heated below visible redness, three inches from glass plate. 

 Thermometer in contact with outer surface at its centre. 



lO > 



In these experiments, the outer surface of the glass, when 

 plain, acquires a temperature at least equal to, and in most cases 

 greater than, when coated with Indian ink. To whatever cause 

 this may be owing, we must hence admit that it may also have 

 been the case in M. De la Roche's experiments. And a second- 

 ary radiation from the outer surface of the screen, which would 

 thus have a greater temperature, would sufficiently account for 

 a greater effect transmitted when the screen was used plain, com- 

 pared with that when the glass was coated with Indian ink ; 

 and thus the supposition of a direct continuance of radiation 

 through the glass is rendered unnecessary. It is true the dif- 

 ference here exhibited is not near so great as that of the trans- 

 mitted effects in M. De la Roche's experiments, but it is suffi- 

 cient to confirm the probability that some such cause may have 

 operated in producing them. 



While upon the subject, I will add a brief remark on the 

 subject of extrenuly thin screens. It has been argued that heat 

 radiates directly through them,, because no difference is percep- 



VOL. XVII. NO. XXXIV. — OCTOBER 1834. R 



