1$ PEOFESSOE TYNDALL ON CALOEESCENCE, 



It follows as a matter of course, that where platinum is raised to whiteness, the com- 

 bustion of wood, charcoal, zinc, and magnesium may also be effected. 



By the arrangement here described, platinum has been raised to redness at a distance 

 of 22 feet from the source of the rays. 



The best mirror, however, scatters the rays more or less ; and by this scattering, the 

 beam at a great distance from the lamp becomes much enfeebled. The effect in free 

 air is intensified when the beam is caused to pass through a tube polished within, 

 which prevents the lateral waste of radiant heat. Such a tube, placed in front of the 

 camera, is represented at A B, fig. 7, Plate I. The flask may be held against its end 

 by the hand, or it may be permanently fixed there. With a battery of fifty cells pla- 

 tinum may be raised to a white heat at the focus of the flask. 



Again, instead of a flask filled with the opaque solution, let a glass or rock-salt lens 

 (L, fig. 8, Plate I.), 2"5 inches wide, and having a focal length of 3 inches, be placed in 

 the path of the reflected beam. The rays are converged ; and at their point of con- 

 vergence all the effects of calorescence and combustion may be obtained. 



In this case the luminous rays are to be cut off by a cell (m n) with plane glass sides ; 

 it may be placed either before or behind the lens. 



Finally, the arrangement shown in Plate I. fig. 9 may be adopted. The beam 

 reflected by the mirror within the camera is received and converged by a second mirror, 

 a/y. At the point of convergence, which may be several feet from the camera, all the 

 effects hitherto described may be obtained. The light of the beam may be cut off at 

 any convenient point of its course ; but in ordinary cases the experiment is best made by 

 employing the bichloride instead of the bisulphide of carbon, and placing the cell (m n) 

 containing the opaque solution close to the camera. The moment the coal points are 

 ignited, explosion, combustion, or calorescence, as the case may be, occurs at the focus. 



The ordinary lamp and camera of Duboscq may be employed in these experiments. 

 With proper mirrors, which are easily procured, a series of efiects which, I ventui-e to 

 affirm, will interest everybody who witnesses them may, with the greatest facility, be 

 obtained. 



It is also manifest that, save for experiments made in darkness, the camera is not 

 necessary. The mirrors and filter may be associated with the naked lamp. 



I have sought to fuse platinum with the invisible rays of the electric light, but hitherto 

 without success. In some experiments I have employed a large model of Foucault's 

 lamp, which permitted me to use a battery of 100 cells. In other experiments I employed 

 two batteries, one of 100 cells and one of 70, making use of two lamps, two mirrors, 

 and two filters, and converging the heat of both lamps in opposite directions upon the 

 same point. When a leaf of platinum was placed at the common focus, the converged 

 beams struck it at opposite sides, and raised it to dazzling whiteness. I am persuaded 

 that the metal could be fused, if the platinum black upon its surface could be retained. 

 But this was immediately dissipated by the intense heat, and, the reflecting power of 

 the metal coming into play, the absorption was so much lowered that fusion was not 



