436 Prof. Tyndall on Calorescence. 



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

 refractory metals, to the rendering of refractory bodies incandes- 

 cent, the step was immediate. To avoid waste by conduction, 

 it was necessary to employ the metals in plates as thin as pos- 

 sible. A few preliminary experiments with platinum-foil, which 

 resulted in failure, raised the question 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 radia- 

 tion. 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 sufficient to compensate 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 even three, inches 

 from the carbon-points. 



The last-mentioned distance allowed me to introduce between 



made at the time. A glass globe, 3f inches in diameter, was filled with 

 the opake solution, and placed in front of the electric light. An intense 

 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 f ths of an inch thick. The space between the plates was filled with 

 the solution, and thus an opake lens was formed. Paper was fired by this 

 lens. In none of these cases, however, could the paper be caused to blaze. 

 Hollow plano-convex lenses filled with the solution were not effective, the 

 focal length of those at my disposal being too great. 



Mr. Mayall was so extremely obliging as to transfer his great photogra- 

 phic 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 will, 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 



