300 KADIATION. 



foundation in fact, the dark solar rays would liave been the philosopher's chief 

 ag-ents of combustion. On a small scale we can readily produce, with the purely 

 invisible rays of the electric light, all that Archimedes is said to have performed 

 with the sun's total radiation. Placing behind the electric light a small concave 

 mirror, the rays are converged, the cone of reflected rays and their point of 

 convergence being rendered clearly visible by the dust always floating in the 

 air. Interposing between the lunnnous focus and the source of rays our solution 

 of iodine, the light of the cone is entirely cut away, but the intolerable heat 

 experienced when the hand is placed, even for a moment, at the dark focus, sliovvs 

 that the calorific rays pass unimpeded through the opaque solution. 



Almost anything that ordinary fire can effect may be accomplished at the focus 

 of invisible rays, the air at the focus remaining at the same time perfectly cold, 

 on account of its transparency to the heat-rays. An air thermometer with a hol- 

 low rock-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. Tlie ether at the focus, 

 and not the air, is the substance 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; cliips of wood are speedily burnt up; lead, tin, and zinc are fused; and 

 discs of charred paper are raised to vivid incandescence. It might be supposed that 

 tlie obscure rays would show no preference for black over white, but they do show 

 a preference, and to oV)taiu rapid combustion, the body, if not already black, ought 

 to be blackened. When metals are to be burned, it is uecessaiy to blacken or 

 otherwise tarnish them, so as to diminish their reflective power. Blackened zinc 

 foil, wliL-n brought into the focus of invisible rays, is instantly caused to blaze, 

 and burns with its peculiar purple flame. Magnesium wire flattened, or tarnished 

 magnesium ribbon, also bursts into splendid combustion. Pieces of charcoal sus- 

 pended in a receiver full of oxygen are also set on fire, 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 oxy- 

 gen, the charcoal be suspended in vacuo, it immediately glows at the place where 

 the focus falls. 



VIII. — TRANSMUTATIOl^ OF RATS : CALORESCEKCE.* 



Eminent experimenters were long occupied in demonstrating the substantial 

 identity of light and radiant heat, and we have now the means of offering a new 

 and striking proof of this identity. . A concave mirror produces beyond the 

 object which it reflects an inverted and magnified image of the object; with- 

 drawing, for example, our iodine solution, an intensely luminous inverted image of 

 the carl>on points of the electric light is formed at the focus of the mirror employed 

 in the foregoing experiments. "VVhen the solution is interposed and the light is 

 cut away, what becomes of this image ? It disappears from sight, but an invisi- 

 ble thermograph remains, and it is only the peculiar constitution of our eyes that 

 disqualifies us from seeing the picture formed Ijy the calorific rays. Falling on 

 white paper, the image chars itst^lf out ; falling on black paper, two holes are 

 pierced in it, corresponding to the images of the two coal points ; but falling on 

 a thin plate of carbon in vacuo, or upon a thin sheet of platinized })lantiuum, 

 either in vacuo or in air, radiant heat is converted into light, and the image stamps 

 itself in vivid incandescence upon bcnh the carbon and the metal. Results sinnlar 

 to those obtained with the electric light have also been obtained with the invisi- 

 ble rays of the lime light and of the sun. 



Before a Cambridge audience, it is hardly necessary to refer to the excellent 

 researches of Professor Stokes at the opposite end of the specti'um. The above 



* I borrow this term trom Professor Cliallis, Philosophical Magazine, \o\. xii., p. 521. 



