6 z CALORESCENCE. PART i. 



found to lie beyond the extreme red, but the augmenta- 

 tion of intensity was so sudden and enormous as far to 

 exceed the maximum heat of the sun previously deter- 

 mined by Professor Miiller. Aqueous vapour powerfully 

 absorbs radiant heat ; so a solar spectrum beyond the 

 earth's atmosphere might probably exhibit as great 

 intensity as the electrical light. With the apparatus 

 described oxidizable substances burst into the flame of 

 common combustion when put into the focus ; but when 

 the chemical action of the oxygen of the atmosphere 

 was excluded by igniting substances in vacuo by the 

 invisible rays of heat, their periods of vibration were so 

 changed as to bring them within the limits of vision. 

 When the electric light is very powerful, a plate of 

 platinized platinum in vacuo is raised to white heat at 

 the focus of invisible rays ; and when the incandescent 

 platinum is looked at through a prism, its light yields 

 a complete and brilliant spectrum. c In all these cases 

 we have a perfectly invisible image of the charcoal 

 points formed by the mirror ; and no experiment illus- 

 trates the change of heat into light ' more strongly 

 than the following : When the plate of platinum or 

 one of charcoal is placed in the focus, the invisible 

 image raises it to incandescence, and thus prints itself 

 visibly on the plate. On drawing the coal points of the 

 lamp apart, or causing them to approach each other, 

 the thermograph follows their motion. By cutting the 

 plate of carbon along the boundary of the thermograph, 

 a second pair of coal points may be formed of the same 

 shape as the original ones, but turned upside down ; 

 and thus by the rays of the one pair of coal points 

 which are incompetent to excite vision, we may cause a 

 second pair to emit all the rays of the spectrum. 

 /Fluorescence and calorescence act in contrary directions. 

 j Fluorescence causes the molecules of a fluorescent sub- 

 I stance to oscillate in slower periods than the incident 



