16 PEOFESSOE TTNDALL ON CALOEESCENCE. 



without a flaw. The most convenient distance of the focus from the source, I find to be 

 about 5 inches ; and the position of the mirror ought to be arranged accordingly. This 

 distance permits of the introduction of an iodine-cell of sufficient depth, while the heat 

 at the focus is exceedingly powerful. 



The isolation of the luminiferous ether from the air is strikingly illustrated by these 

 experiments. The air at the focus may be of a freezing temperature, while the ether 

 possesses an amount of heat competent, if absorbed, to impart to that air the temperature 

 of flame. An air-thermometer is unaffected where platinum is raised to a white heat. 

 Numerous experiments will suggest themselves to every one who wishes to operate upon 

 the invisible heat-rays. The dense volumes of smoke which rise from a blackened block 

 of wood when it is placed in the dark focus are very striking : matches are of course at 

 once ignited, and gunpowder instantly exploded. Dry black paper held there bursts into 

 flame. Chips of wood are also inflamed : the dry wood of a hat-box is very suitable for 

 this experiment. When a sheet of browTi paper is placed a little beyond the focus, it is 

 first brought to vivid incandescence over a large space ; the paper then yields, and the 

 combustion propagates itself as a burning ring round the centre of ignition. Charcoal is 

 reduced to an ember at the focus, and disks of charred paper glow with extreme vivid- 

 ness. Sheet lead and tin, if blackened, may be fused, while a thick cake of fusible metal 

 is quickly pierced and melted. Blackened zinc foil placed at the focus bursts into flame ; 

 and by drawing the foU slowly through the focus, its ignition may be kept up till the 

 whole of the foil is consumed. Magnesium wire, flattened at the end and blackened, 

 also bursts into vivid combustion. A cigar or a tobacco-pipe may of course be instantly 

 lighted at the dark focus. The bodies experimented on may be enclosed in glass 

 receivers, the concentrated rays will still burn them after having crossed the glass. A 

 small chip of wood in a jar of oxygen bursts suddenly into flame; charcoal bums, while 

 charcoal bark throws out suddenly showers of scintillations. 



§6. 



In all these cases the body exposed to the action of the invisible rays was more or less 

 combustible. It required to be heated more or less to initiate the attack of the atmo- 

 spheric oxygen. Its vividness was in great part due to combustion, and does not furnish 

 a conclusive proof that the refrangibility of the incident rays was elevated. This, which 

 is the result of greatest theoretic import, is effected by exposing non-combustible bodies 

 at the focus, or by enclosing combustible ones in a space devoid of oxygen. Both in air 

 and in vacuo platinized platinum foil has been repeatedly raised to a white heat. The 

 same result has been obtained with a sheet of charcoal or coke suspended in vacuo. On 

 looking at the white-hot platinum through a prism of bisulphide of carbon, a rich and 

 complete spectrum was obtained. All the colours, from red to violet, glowed with 

 extreme vividness. The waves from which these colours were primaiily extracted had 

 neither the visible nor the extra-violet rays commingled with them ; they were exclusively 

 extra-red. The action of the atoms of platinum, copper, silver, and carbon upon these 



