20 Professor TymJall [Jan, 17, 



attached to one of its ends, while the other end is connected with a 

 gas-pipe of the Institution. From three different points of the copper 

 wire branch wires pass into three little models of houses, where 

 the branches are subdivided and furnished with spirals of i^latinum 

 wire. The branches reunite afterwards in the gas-pipe. When the 

 branch currents pass through the houses, they kindle the platinum 

 lamps, which glow with a soft, white light. This, if I understand 

 aright, is Mr. Edison's proposed mode of illumination. The electric 

 force is at hand. Metals sufficiently refractory to bear being raised 

 to vivid incandescence are also at hand. The principles which 

 regulate the division of the current and the development of its 

 light and heat are perfectly well known. There is no room for a 

 ' ' discovery," in the scientific sense of the term, but there is ample 

 room for the exercise of that mechanical ingenuity which has given 

 us the sewing machine and so many other useful inventions, and 

 which engages a greater number of minds in the United States than 

 in any other nation in the world.* 



It is sometimes stated as a recommendation to the electnc light, 

 that it is light without heat ; but to disprove this, it is only necessary 

 to point to the experiments of Davy, which showed that the heat of 

 the Voltaic arc transcends 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, through the gradual 

 augmentation of the latter, raised to a white heat. When fii*st 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 1000° Fahr. does it begin to glow with 

 a faint, red light. The rays which it emits prior to redness are all in- 

 visible rays, which can warm the hand but which 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 

 dazzling platinum wire. They make up 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 u^jon 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 simple gases and metalloids — of oxygen, 

 hydrogen, nitrogen, chlorine, iodine, bromine, suljjhur, phosphorus, 

 and even carbon, for the invisible heat-rays is extraordinary. Dis- 



* Knowing something of the intricncy of the problem, I should certainly 

 prefer seeing it in Mr. Edison's bauds than in mine. It may be added tbat more 

 than tliirty years ago the radiation from incandescent platinum was admirably 

 investigated by Dr. Draper of New York. 



