450 FKAGMENTS 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 in- 

 visible 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 temperature of 

 the wire is raised to whiteness, these dark rays not only 

 persist, but they are enormously augmented in inten- 

 sity. 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 foun- 

 dation. 



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, phosphorus, 

 and even of carbon, for the invisible heat rays is extra- 

 ordinary. 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 remaining trans- 

 parent 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 



