22 
ON COMBUSTION BY INVISIBLE BAYS. 
containing the opaque 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 may be produced:— 
When a piece of black paper is placed in the focus, it is pierced by the invisible rays, 
as if a white-hot spear had been suddenly driven through it. The paper instantly 
blazes, without apparent contact with anything 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. 
Disks of charred paper placed at the focus are raised to brilliant incandescence; char¬ 
coal 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. 
A mixture of oxygen and hydrogen is exploded in the dark focus, through the ignition 
of its envelope. & 
A strip of blackened zinc-foil placed at the focus is pierced and inflamed by the 
invisible rays. By gradually drawing the strip through the focus, it may be kept 
blazing with its characteristic purple light for a considerable time. This experiment is 
particularly beautiful. 
Magnesium wire, presented suitably to the focus, burns with almost intolerable bril¬ 
liancy. 
The effects thus far described are, in part, due to chemical action. The substances 
placed at the dark focus are oxidizable ones, which, when heated sufficiently, are 
attacked by the atmospheric oxygen, ordinary combustion being the result. But the 
experiments may be freed from this impurity. A thin plate of charcoal, placed in vacuo , 
is raised to incandescence at the focus of invisible rays. Chemical action is here entirely 
excluded. A thin plate of silver or copper, with its surface slightly tarnished by the 
sulphide of the metal, so as to diminish its reflective power, is raised to incandescence 
either in vacuo or in air. With sufficient battery-power and proper concentration, a plate 
of platinized platinum is rendered white-hot at the focus of invisible rays; and when the 
incandescent platinum is looked at through a prism, its light yields a complete and 
brilliant spectrum. In all these cases we have, in the first place, a perfectly invisible image 
of the coal points formed by the mirror; and no experiment hitherto made illustrates the 
identity of light and heat more forcibly than this one. When the plate of metal or of 
chaicoal is placed at the focus, the invisible image raises it to incandescence, and thus 
prints itself visibly upon the plate. On drawing the coal points apart, or on causing 
them to approach each other, the thermograph of the points follows their motion. By 
cutting the plate of carbon along the boundary of the thermograph, we might obtain a 
second pair of coal points, of the same shape as the original ones, but turned upside 
down ; and thus by the rays of the one pair of coal points, which are incompetent to 
excite vision, we may cause a second pair to emit all the rays of the spectrum. 
The ultra-red radiation of the electric light is known to consist of ethereal undulations 
ot greater length, and slower periods of recurrence, than those which excite vision. 
When, therefore, those long waves impinge upon a plate of platinum, and raise it to 
incandescence, their period of vibration is changed. The waves emitted by the platinum 
are shortei and of more rapid recurrence than those falling upon it; the refrangibility 
being thereby raised, and the invisible rays rendered visible. Thirteen years ago, Pro- 
lessor Stokes proved that by the agency of sulphate of quinine, and various other sub¬ 
stances, the ultra-violet rays of the spectrum could be rendered visible. These invisible 
lays of high refrangibility, impinging upon a proper medium, cause the molecules of 
that medium to oscillate in slower periods than those of the incident waves. In this 
case, therefore, the invisible rays are rendered visible by the lowering of their refrangi¬ 
bility; while in the experiments of the speaker, the ultra-redrays are rendered visible by 
the raising of their refrangibility. To the phenomena brought to light by Professor 
