HEAT. 49 



as will be presently explained, would seem to indicate that 

 the heat was rendered more intense by condensation in 

 the solid matter, as water is in this case decomposed by a 

 heated body, which body has itself been heated by the com- 

 bining elements of water. The apparent effect, however, of 

 the introduction of solid incombustible matter into heated gas, 

 is a conversion of heat into light. 



Dr. Tyndall, by passing light from the voltaic arc through 

 solutions of iodine, separates the invisible rays of heat from 

 the luminous rays, and then reproduces light by receiving the 

 former on platinum foil. 



If we concentrate into a focus by a large lens a dim light, 

 we increase the intensity of the light. Now, if a heated body 

 be taken, which, to the unassisted eye, has just ceased to be 

 visible, it seems probable that by collecting and condensing 

 by a lens the different rays which have so ceased to be visible, 

 light would reappear at the focus. The experiment is, for 

 reasons obvious to those acquainted with optics, a difficult one, 

 and to be conclusive, should be made on a large scale, and 

 with a very perfect lens of large diameter and short focus. I 

 have obtained an approximation to the result in the following 

 manner : In a dark room a platinum wire is brought just to 

 the point of visible ignition by a constant voltaic battery ; it 

 is then viewed, at a short distance, through an opera-glass of 

 large aperture applied to one eye, the other being kept open. 

 The wire will be distinctly visible to that eye which regards it 

 through the opera-glass, and at the same time totally invisible 

 to the other and naked eye. It may be said with some jus- 

 tice that such experiments prove little more than the fact 

 already known, viz. that by increasing the intensity of heat, 

 light is produced ; they however exhibit this effect in a 

 more striking form, as bearing on the relations of heat and 

 light. 



With regard to chemical affinity and magnetism, perhaps 

 the only method by which in strictness the force of heat may 

 be said to produce them is through the medium of electricity, 

 the thermo-electrical current, produced, as before described, 

 by heating dissimilar metals, being capable of deflecting the 

 magnet, of magnetising iron, and exhibiting the other mag- 



E 



