Prof. Tyndall on Combustion by Invisible Rays. 243 



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, flattened out and blackened, 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 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. Che- 

 mical 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 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 boundary 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 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 sethereal undulations of greater length, and slower periods of re- 

 currence, than those which excite vision. When, therefore, those 

 long waves impinge upon a plate of platinum and raise it to incan- 

 descence, their period of vibration is changed. The waves emitted 

 by the platinum are shorter, and of more rapid recurrence, than 

 those falling upon it, the refrangibility being thereby raised, and the 

 invisible rays rendered visible. Thirteen years ago Professor Stokes 

 published the celebrated discovery that by the agency of sulphate of 

 quinine, and various other substances, the ultra-violet rays of the 

 spectrum could be rendered visible. These invisible rays 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 ren- 

 dered visible by the lowering of their refrangibility ; while in the 

 experiments of the lecturer, the ultra-red rays are rendered visible by 

 the raising of their refrangibility. To the phenomena brought to 

 light by Professor Stokes, the term fluorescence has been applied by 



