PEOFESSOE TTNDALL ON CALOEESCENCE, 11 



bustion and fusion which were then obtained with the invisible rays of the electric light 

 and of the sun *. 



From the setting of paper on fire, and the fusion of non-refractory metals, to the 

 rendering of refractory bodies incandescent, the step was immediate. To avoid waste by 

 conduction, it was necessary to employ the metals in plates as thin as possible. A few 

 preliminary experiments with platinum foil, which resulted in failure, raised the ques- 

 tion whether, even with the total radiation of the electric light, it would be possible to 

 obtain incandescence without combustion. Abandoning the use of lenses altogether, 

 I caused a thin leaf of platinum to approach the ignited coal points. It was observed 

 by myself from behind, while my assistant stood beside the lamp, and, looking through 

 a dark glass, observed the distance between the platinum foil and the electric light. At 

 half an inch from the carbon points the metal became red-hot. The problem now was 

 to obtain, at a greater distance, a focus which should possess a heating-power equal to 

 that of the direct rays at a distance of half an inch. 



In the first attempt the direct rays were utilized as much as possible. A piece of 

 platinum foil was placed at a distance of an inch from the carbon points, there receiving 

 the direct radiation. The rays emitted backwards from the points were at the same time 

 converged by a small mirror upon the foil, and were found more than sufiicient to com- 

 pensate for the diminution of intensity due to the withdrawal of the foil to the distance 

 of an inch. By the same method incandescence was subsequently obtained when the 

 foil was removed two, and three, inches from the carbon points. 



The last-mentioned distance allowed me to introduce between the focus and the 

 source of rays a cell containing the solution of iodine. The transmitted obscure rays 

 were foimd of sufficient power to inflame paper, or to raise platinum foil to incan- 

 descence. 



These experiments, however, were not unattended with danger. The bisulphide of 



* To the experiments there described the following may be added, as made at the time. A glass globe, 

 3| inches in diameter, was filled with the opaque solution, and placed in front of the electric light. An in- 

 tense focus of invisible rays was formed immediately beyond the globe. Black paper held in this focus was 

 pierced, a burning ring being produced. A second spherical flask, 9 inches in diameter, was filled with the 

 solution and employed as a lens. The effects, however, were less powerful than those obtained with the smaller 

 flask. 



Two plano-convex lenses of rock-salt, 3 inches in diameter, were placed with their flat surfaces opposite, but 

 separated from each other by a brass ring ^ths of an inch thick. The space between the plates was filled with 

 the solution, and thus an opaque lens was formed. Paper was fired by this lens. In none of these cases, how- 

 ever, could the paper be caused to blaze. Hollow plano-convex lenses filled with the solution were not efiective, 

 the focal length of those at my disposal being too great. 



Mr. Mayat.l was so extremely obliging as to transfer his great photographic camera from Brighton to London, 

 for the purpose of enabling me to operate with the fine glass lens, 20 inches in diameter, which belonged to it : 

 the result was not successful. It wDl, however, be subsequently shown that both the hollow lens and the 

 glass lens are effective when, instead of the divergent rays of the electric lamp, we employ the parallel rays of 

 the sun. 



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