THE WLKGTRtC 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. 
