PEOFESSOE TTNDALL ON CALOEESCENCE. 19 



effected. By coating the platinum with lampblack it has been brought to the verge of 

 fusion, the incipient yielding of the mass being perfectly apparent after it had cooled. 

 Here, however, as in the case of the platinized platinum, the absorbing substance dis- 

 appears too quickly. Copper and aluminum, however, when thus ti-eated, are speedily 

 burnt up. 



§8. 



Thus far I have dealt exclusively with the invisible radiation of the electric light ; 

 but all solid bodies raised to incandescence emit these invisible calorific rays. The 

 denser the incandescent body, moreover, the more powerful is its obscure radiation. 

 We possess at the Royal Institution very dense cylinders of lime for the production of 

 the Drummond light; and when a copious oxyhydrogen-flame is projected against one 

 of them it shines with an intense yellowish light, while the obscure radiation is exceed- 

 ingly powerful. Filtering the latter from the total emission by the solution of iodine, all 

 the effects of combustion and calorescence described in the foregoing pages may be 

 obtained at the focus of the invisible rays. The light obtained by projecting the oxyhy- 

 drogen-flame upon compressed magnesia, after the manner of Signer Carlevaris, is 

 whiter than that emitted by our lime ; but the substance being light and spongy, its 

 obscure radiation is surpassed by that of our more solid cylinders*. 



The invisible rays of the sun have also been transmuted. A concave mirror, 3 feet in 

 diameter, was mounted on the roof of the Royal School of Mines in Jermyn Street, 

 The focus was formed in a darkened chamber in which the platinized platinum foil 

 was exposed. Cutting off the visible rays by the solution of iodine, feeble but distinct 

 incandescence was there produced by the invisible rays. 



A blackened tin tube (A B, fig. 10, Plate I.) with square cross section and open at 

 one end, was furnished at the other with a plane mirror {xy) forming an angle of 45° 

 with the axis of the tube. A lateral aperture {x o), about 2 inches square, was cut out 

 in front of the mirror. Over this aperture was placed a leaf of platinized platinum. 

 Turning the leaf towards the concave mirror, the concentrated sunbeams were permitted 

 to fall upon it. In the glare of daylight it was quite impossible to see whether the 

 platinum was incandescent or not ; but placing the eye at B, the glow of the platinum 

 could be seen by reflexion from the plane mirror. Incandescence was thus obtained at 

 the focus of the large mirror, XY, after the removal of the visible rays by the iodine 

 solution, mn. 



* The discovery of fluorescence by Professor Stokes naturally excited speculation as to the possibility of a 

 change of refrangibUity la the opposite direction. Mr. Geove, I believe, made various experiments with a view 

 to effect such a change ; but very soon after the publication of Professor Stokes's Memoir Dr. Milleb pointed 

 to the lime-light itself as an instance of raised refrangibility. From its inability to penetrate glass screens, he 

 inferred that the radiation of the oxyhydrogen-flame was almost wholly extra-red, an inference the truth of 

 which has been since established by direct prismatic analysis. The intense light produced by the oxj'hydrogen- 

 flame when projected upon lime must, he concluded, involve a change of period from slow to quick, or, in other 

 words, a virtual elevation of refrangibility. (Elements of Chemistry, 1855, p. 210.) 



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