34 Prof. Tyndall Invisible Radiation of Electric Light. [Jan. 19, 



the limits of the atmosphere, would probably exhibit as steep a peak as 

 that of the electric light. 



In the experiments now to be referred to, the rays from the electric light 

 were converged by a small concave mirror. The glass mirror silvered at the 

 back, which usually accompanies the camera of Duboscq's electric lamp, 

 was one of the first employed. It was brought so near the electric light 

 as to cast an image of the coal-points five or six inches in advance of the 

 light. A solution of iodine in bisulphide of carbon, contained in a rock- 

 salt cell, was then placed in front of the lamp : the light was thereby cut 

 off ; but the focus of dark rays remained, and various effects of combustion 

 and incandescence were obtained at the focus. A mirror 4 inches in dia- 

 meter, and silvered in front, will enable an experimenter to obtain most, if 

 not all the results now to be mentioned. I also employ a mirror 8 inches 

 in diameter, and having a focal length of 8 inches, with excellent effect. 



It is not necessary to enclose the opaque solution in a rock-salt cell. 

 The vessel intended for a solution of alum, which usually accompanies the 

 lamp of Duboscq, and the sides of which are of glass, answers admirably. 

 It is, however, not quite deep enough for the several tests to which I have 

 subjected it, and in crucial experiments I employ a deeper vessel with rock- 

 salt sides. 



With the 8-inch mirror just referred to behind the electric light, the 

 opaque solution in front, and the focus of invisible rays about 6 inches 

 distant from the electric light, the following effects have been obtained : 



1 . Wood, painted black, when brought into the dark focus, emits copious 

 volumes of smoke, and is soon kindled at the two spots on which the images 

 of the two coal-points fall. 



2. A piece of brown paper placed near the focus soon shows a burning 

 surface, which spreads over a considerable space, the paper finally bursting 

 into flame. 



3. Black paper brought into the focus is immediately inflamed. 



4. The wood of a hat-box similarly placed is rapidly burnt through, 

 and usually bursts into flame. 



5. The end of a cigar, placed at the dark focus, is instantly ignited. 



6. Disks of charred paper placed in the focus are raised to brilliant 

 incandescence, surfaces of considerable extent being brought to a vivid 

 glow. Charcoal is also ignited. 



7. A piece of charcoal, suspended in a receiver of oxygen, is ignited in 

 the dark focus and caused to burn brilliantly, the rays after crossing the 

 glass of the receiver being still sufficiently powerful to heat the coal up to 

 incandescence. 



8. A mixture of oxygen and hydrogen is exploded in the dark focus by 

 the ignition of its envelope. 



9. A piece of zinc foil, blackened on one side to diminish reflexion, is 

 pierced and inflamed. By gradually drawing the strip, once inflamed, 

 across the focus, it may be kept blazing for a considerable length of time. 

 This is a particularly beautiful experiment. 



