THE ELECTRIC LIGHT. 683 



which has given us the sewing machine and so many other 

 useful inventions. Knowing something of 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 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 

 degrees 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 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 radiation 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 extraordinary. 

 Dissolved in a proper vehicle, iodine cuts the visible 

 radiation sharply off, but allows the invisible free trans- 

 mission. 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 



*More than thirty years ago the radiation from incandescent plati- 

 num was admirably investigated by Dr. Draper of New York. 



