242 Royal Institution:- — 



visible rays as that exhibited by the electric light, which is practically 

 uninfluenced by atmospheric absorption. 



Having thus demonstrated that a powerful flux of dark rays 

 accompanies the bright ones of the electric light, the question arises, 

 Can we separate the one class of rays from the other ? 



In the lecturer's first experiments on the invisible radiation of 

 the electric light, black glass was the substance made use of. The 

 specimens, however, which he was able to obtain, destroyed, along 

 with the visible, a considerable portion of the invisible radiation. 

 But the discovery of the deportment of elementary gases directed 

 his attention to other simple substances. He examined sulphur 

 dissolved in bisulphide of carbon, and found it almost perfectly trans- 

 parent to the invisible rays. He also examined the element bromine, 

 and found that, notwithstanding its dark colour, it was eminently 

 transparent to the ultra-red rays. Layers of this substance, for ex-, 

 ample, which entirely cut off the light of a brilliant gas-flame, 

 transmitted its invisible radiant heat with freedom. Finally he 

 tried a solution of iodine in bisulphide of carbon, and arrived at the 

 extraordinary result that a quantity of dissolved iodine sufficiently 

 opake to cut off the light of the midday sun was, within the limits 

 of experiment, absolutely transparent to invisible radiant heat. 



This, then, is the substance by which the invisible rays of the 

 electric light may be almost perfectly detached from the visible ones. 

 Concentrating by a small glass mirror, silvered in front, the rays 

 emitted by the carbon-points of the electric lamp, we, obtain a con- 

 vergent cone of light. Interposing in the path of this concentrated 

 beam a cell containing the opake solution of iodine, the light of the 

 cone is utterly destroyed, while its invisible rays are scarcely, if at 

 all, meddled with. These converge to a focus, at which, though 

 nothing can be seen even in the darkest room, the following series 

 of effects has been produced : — 



A piece of black paper placed in the focus, is pierced by the in- 

 visible rays, as if a white-hot spear had been suddenly driven through 

 it. The paper instantly blazes, without apparent contact with any- 

 thing hot. 



A piece of brown paper placed at the focus soon shows a red-hot, 

 burning surface, extending over a considerable space of the paper, 

 which finally bursts into flame. 



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

 A pile of wood and shavings, on which the focus falls, is quickly 

 ignited, and thus a fire may be set burning by the invisible rays. 



A cigar or a pipe is immediately lighted when placed at the focus 

 of invisible rays. His Royal Highness the Comte de Paris performed 

 this experiment at the lecture. 



Disks of charred paper placed at the focus are raised to brilliant 

 incandescence ; charcoal is also ignited there. 



A piece of charcoal, suspended in a glass receiver full of oxygen, 

 is set on fire at the focus, burning with the splendour exhibited by 

 this substance in an atmosphere of oxygen. The invisible rays, 

 though they have passed through the receiver, still retain sufficient 

 power to render the charcoal within it red-hot. 



