366 



RADIANT HEAT. 



at the distance of about three inches apart, were exposed first with, 

 and then without, a screen of plate glass one-eighth of an inch thick. 

 The mode of observation was to note the rise of the thermometer 

 blackened with China ink, in the time which it occupied the white- 

 washed thermometer to rise 1° Fahrenheit. 



A strong and curious confirmation of the heating power derived 

 from luminiferous rays simply, has been furnished in a fact mentioned 

 to me by Dr. Bennett, Professor of Physiology in the University of 

 Edinburgh, viz: that in the exhibition of anatomical subjects by 

 means of the oxy-hydrogen microscope, the light concentrated by a 

 lens, at a few inches distance from the source, had so energetic an 

 action as to burn up and destroy the specimens placed in the focus. 



On the Theoretical Explanation of some former Experiments. 



The experiments of Melloni (of which some general abstracts are 

 given in my former report (1840, p. 335,)) certainly seem at first sight 

 to present some anomalous results. 



The main question which seems to arise is, whether the effects of 

 heat, as compared with those of illumination, do not follow such 

 widely discrepant laws as to make it difficult to ascribe them to the 

 same set of waves; and this both with regard to terrestrial and to the 

 solar heat. 



To take a single instance: rock-salt is said to be analogous, for heat, 

 to colorless media for light; alum is described as totally impermeable 

 to " dark heat,"' and partially so to the rays from a lamp, that is, it 

 may be wholly impermeable to those rays of the lamp which are iden- 

 tical with "dark heat" (Species I) in their relation to absorption, ac- 

 cording to the texture of surfaces, and wholly permeable to those 

 (Species II) which are associated with light, and produce this effect 

 in proportion to the absorption of light by dark colored surfaces; or in 

 the language of the wave-theory, wholly impermeable to rays of 

 longer wave-length, and wholly permeable to certain rays of smaller 

 wave-length. 



