470 FRAGMENTS OF SCIENCE 



the intricacy of the practical problem, I should certainly 

 prefer seeing it in Mr. Edison's hands to having it in 

 mine. ' 



It is sometimes stated as a recommendation to the elec- 

 tric light that it is light without heat; but to disprove 

 this it is only necessary to point to the experiments of 

 Davy, which show that the heat of the voltaic arc tran- 

 scends that of any other terrestrial source. The emission 

 from the carbon points is capable of accurate analysis. 

 To simplify the subject, we will take the case of a plati- 

 num 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 un- 

 til 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 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 accompani- 

 ment. 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 radiation and 



1 More than thirty years ago the radiation from incandescent platinum was 

 admirably investigated by Dr. Draper of New York. 



