64 CORRELATION OF PHYSICAL FOKCES. 



be made to produce luminous effects, though I am not aware 

 that the experiment has ever been made. 



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- visi- 

 ble, 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 bo made on a large scale, 

 and with a very perfect lens of large diameter and short lo- 

 cus. 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 vol- 

 taic 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 justice that such experiments prove little more 

 than the fact already known, viz. that by increasing the in- 

 tensity of heat, light is produced : they however exhibit this 

 effect in a more striking form, as bearing on the relations of 

 heat and light. 



AVith 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- 

 netic effects, and also of forming and decomposing chemical 

 compounds, and this in proportion to the progression of heat : 

 this has not, indeed, as yet been proved to bear a measurable 

 quantitative relation to the other forces thus produced by it, 

 because so little of the heat is utilised or converted into eleo 



