344 RADIANT HEAT. 



The author gives, as an illustration, the following scale of different 

 kinds of heat, in the order of refrangibility, beginning with the 

 lowest : 



1. Heat from ice. 



2. Heat from the hand. 



3. Heat from boiling water. 



4. Heat from a vessel of mercury under its boiling temperature. 



5. Heat from metal, smoked; wholly non-luminous in the dark, 

 heated by an alcohol lamp behind it. 



6. Heat from incandescent platina, (over a spirit lamp.) 



7. Heat from an oil lamp, (direct.) 



8. Oil-lamp heat transmitted by common mica. 



9. Oil-lamp heat transmitted by glass, (argand lamp.) 



10. Oil-lamp heat transmitted by citric acid. 



11. Oil-lamp heat transmitted by alum. 



12. Oil-lamp heat transmitted by ice. 



Melloni having shown that a portion of the heat from a luminous 

 source is transmitted through certain screens, which are wholly opaque 

 to light, it became natural to inquire whether the rays so passed pos- 

 sessed the properties of heat from dark sources. This he found to be 

 partly the case and partly not. 



The direct test of examining the refrangibility of the heat-rays 

 issuing from the screen occurred to Professor Forbes, who found that 

 opaque glass and mica act as clear glass and mica do in elevating the 

 mean refrangibility of the transmitted heat, an action analogous to that 

 of yellow glass upon light. — (See 3d Series, art. 73, 81, &c.) 



But in all this there was nothing exactly equivalent to the action 

 of red glass; this, however, was discovered by Melloni, by the happy 

 suggestion of covering the surface of rock salt with smoke. 



These remarks introduce more clearly the main object of Professor 

 Forbes in following up the inquiry. In the present paper the details 

 of many series of experiments are given, and the more precise results 

 now established may be stated as follows: 



I. The peculiar character of the film of smoke on the surface of a 

 diathermanous medium, analogous to redness in glass for light, was 

 found to be possessed by — 1. The simple powder of charcoal. 2. 

 Some other dull earthy powders. 3. Surfaces simply dull, or devoid 

 of polish. 4. Surfaces irregularly furrowed, as with emery or sand- 

 paper. 5. Polished surfaces, on which fine distinct lines have been 

 drawn. G. Transparent mica, when mechanically laminated, which, 

 as a continuous medium, possesses opposite properties. 



II. All kinds of heat (i. e., of all refrangibilities) seem affected 

 indifferently by the following media: 



1. The thinnest leaf-gold, whieh is impervious to any hind of heat. 



2. Fine metallic gratings, which transmit all kinds of heat in a pro- 

 portion probably exactly that of the areas of their interstices. 



3. Thread gratings. 



4. Most crystalline bodies in a state of powder, in which case they 

 approximate to a condition of opacity for heat. 



III. The following substances, in addition to those before known, 



