RADIATION. 47 



at the focus remaining at the same time perfectly cold, 

 on account of its transparency to the heat-rays. An air 

 thermometer, with a hollow ratck-salt bulb, would be 

 unaffected by the heat of the focus : there would be no 

 expansion, and in the open air there is no convection. 

 The aether at the focus, and not the air, is the sub- 

 stance in which the heat is embodied. A block of wood, 

 placed at the focus, absorbs the heat, and dense volumes 

 of smoke rise swiftly upwards, showing the manner in 

 which the air itself would rise, if the invisible rays were 

 competent to heat it. At the perfectly dark focus dry 

 paper is instantly inflamed : chips of wood are speedily 

 burnt up : lead, tin, and zinc are fused : and disks of 

 charred paper are raised to vivid incandescence. It 

 might be supposed that the obscure rays would show 

 no preference for black over white ; but they do show 

 a preference, and to obtain rapid combustion, the body, 

 if not already black, ought to be blackened. When 

 metals are to be burned, it is necessary to blacken or 

 otherwise tarnish them, so as to diminish their reflective 

 power. Blackened zinc foil, when brought into the 

 focus of invisible rays, is instantly caused to blaze, and 

 burns with its peculiar purple light. Magnesium wire 

 flattened, or tarnished magnesium ribbon, also bursts 

 into flame. Pieces of charcoal suspended in a receiver 

 full of oxygen are also set on fire when the invisible 

 focus falls upon them ; the dark rays after having 

 passed through the receiver, still possessing sufficient 

 power to ignite the charcoal, and thus initiate the 

 attack of the oxygen. If, instead of being plunged in 

 oxygen, the charcoal be suspended in vacuo, it immedi- 

 ately glows at the place where the focus falls. 



