442 Prof. Tyndall on Calorescence. 



kingly illustrated by these experiments. ' The air at the focus 

 may be of a freezing temperature, while the sether 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 brown 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 vividness. 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 foil 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; char- 

 coal burns, 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. Tt required to be 

 heated more or less to initiate the attack of the atmospheric oxy- 

 gen. 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 ob- 

 tained. All the colours, from red to violet, glowed with extreme 

 vividness. The waves from which these colours were primarily 

 extracted had neither the visible nor the extra-violet rays com- 



