448 FRAGMENTS OF SCIENCE. 



case of a platinum wire at first slightly warmed by the 

 current, and then gradually raised to a white heat. 

 When first warmed, the wire sends forth rays which 

 have no power on the optic nerve. They are what we 

 call invisible rays; and not until the temperature of the 

 wire has reached nearly 1,000 Fahr., does it begin to 

 glow with a faint, red light. The rays which it emits 

 prior to redness are all invisible rays, which can warm 

 the hand but cannot excite vision. When the tem- 

 perature of the wire is raised to whiteness, these dark 

 rays not only persist, but they are enormously aug- 

 mented in intensity. They constitute about 95 per 

 cent, of the total radiation from the white-hot platinum 

 wire. They make up nearly 90 per cent, of the emission 

 from a brilliant electric light. You can by no means 

 have the light of the carbons without this invisible 

 emission as an accompaniment. The visible radiation 

 is, as it were, built upon the invisible as its necessary 

 foundation. 



It is easy to illustrate the growth in intensity of 

 these invisible rays as the visible ones enter the radia- 

 tion and augment in power. The transparency of the 

 elementary gases and metalloids of oxygen, hydrogen, 

 nitrogen, chlorine, iodine, bromine, sulphur, phos- 

 phorus, and even of carbon, for the invisible heat rays is 

 extraordinary. Dissolved in a proper vehicle, iodine 

 cuts the visible radiation sharply off, but allows the 

 invisible free transmission. By dissolving iodine in 

 sulphur, Professor Dewar has recently added to the 

 number of our effectual ray-filters. The mixture may 

 be made as black as pitch for the visible, while remain- 

 ing transparent for the invisible rays. By such filters 

 it is possible to detach the invisible rays from the total 

 radiation, and to watch their augmentation as the light 

 increases. Expressing the radiation from a platinum 



