1824.] Mr. Pcwe/l on Terrestrial Light and Heat. 181 



Article V. 



Remarks on Light and Heat from Terrestrial Sources. By 

 Baden Powell, MA. FRS. 



Jug. 8, 1824. 



(1.) Tn a former paper I made some remarks on the necessity 

 of applying some certain and definite tests in order to discrimi- 

 nate different species of heating effects : and adverted to the 

 application of such tests to the investigation of the nature of 

 solar heat. Similar methods may easily be adopted in order to 

 examine the constitution of the heating effect emitted or radiated 

 from incandescent, and burning bodies. This latter part of the 

 inquiry is one of the greatest interest, and at the same time one 

 which appears to require more particular examination than it 

 has hitherto received. 



The grand question regards the interception of radiant heat 

 from the sources above-mentioned by a glass screen ; and to this 

 point have the well-known researches of Leslie, De la Roche, 

 &c. been directed. Some questions which appeared to me to 

 originate from those investigations, I have attempted to examine 

 experimentally ; and have had the honour of laying an account 

 of them before the Royal Society, though at too late a period of 

 the session (June, 1824) to adinit of its being read before the 

 vacation. It is not, therefore, my intention at present to enter 

 upon any detail of those experiments, but merely to make a few 

 incidental remarks on the design of them, and on the subject in 

 general; and I only allude to the circumstances above-mentioned 

 in order to account for any apparent delay in bringing forward 

 the particulars of the experiments. 



(2.) The object of these inquiries cannot be better elucidated 

 than by taking a very brief review of the present state of 

 op nions on the subject. 



The celebrated experiments of Prof. Leslie on the interceptive 

 power of screens (on Heat, Chap. 3), have most conclusively 

 established that at temperatures not greater than that of boiling 

 water the radiant heat emitted is totally intercepted by a plate 

 of glass ; and that any effect produced on a thermometer 

 beyond it is owing solely to the heat which the screen has 

 acquired and radiates again. His conclusions, however, do not 

 immediately apply to the emission of heat at very high temper- 

 atures. 



That there exist essential differences between the constitution 

 of the heating power of luminous hot bodies, and that of the 

 same power proceeding from those which are non-luminous, was 



