NATURAL PHILOSOPHY. . 207 



Discs of charred paper placed at the focus are raised to brilliant in- 

 candescence ; charcoal is also ignited tlierc. 



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

 set on fire at the focus, burning with the splendor exhibited by this 

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

 they have passed through the receiver, btill 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 envelop. 



A strip of blackened zinc-foil placed at the focus is pierced and in- 

 flamed 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 brilliancy. 



The effects thus far described are, in part, due to chemical action. 

 The substances placed at the dark focus are oxydizable ones, which, 

 when heated sufficiently, are attacked by the atmospheric oxygen, or- 

 dinary combustion being the result. But the experiments may be 

 freed from this impurity. A thin plate of charcoal, placed in vacua, 

 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 concentra- 

 tion, 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 charcoal 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 boun- 

 dary of the thermograph, we may 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 cue pair of toal 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 of greater length, and slower periods of recur- 

 rence, 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 plati- 

 num are shorter, and of more rapid recurrence, than those falling upon 

 it, the refrangibility being thereby raised, and the invisible rays ren- 

 dered visible. Thirteen years ago, Prof. Stokes published the noble 

 discovery that by the agency of sulphate of quinine, and various other 

 substances, the ultra-violet rays of the spectrum could be rendered 

 ^i iLle. These invisible rays of high refrangibility, impinging upon 



