Mr. P. G. Tait on the History of Energy. 55 



3. A piece of zinc-foil placed at the focus of invisible rays, 

 burns with its characteristic purple flame. Chemists know that 

 there is some difficulty in causing this substance to blaze, even 

 in a flame of high temperature. 



4. Placing a thin plate of a refractory metal at this focus, a 

 space of this metal, corresponding to the invisible image, is raised 

 to brilliant incandescence. 



5. When, instead of a metal, a sheet of carbon, placed in 

 vacuo, is brought into the focus of invisible rays, the incan- 

 descent thermograph of the coal-points is also vividly formed. 

 Cutting the sheet of carbon along the boundaries of the ther- 

 mograph, we obtain a pair of incandescent coal-points, larger 

 and less intensely illuminated than the original ones, but of 

 the same shape. Thus, by means of the invisible rays of one 

 pair of coal-points, we may render a second pair luminous. 



6. By a suitable arrangement of the carbon terminals a metal 

 on which their image falls may be raised to a white heat. 



7. The light of a metal thus rendered white-hot yields, on 

 prismatic analysis, a brilliant spectrum, which is derived wholly 

 from the invisible rays lying beyond the extreme red of the source. 



8. When the electric light is looked at, directly, through the 

 solution employed in these experiments, nothing is seen. 



9. When, in a dark room, a suitable screen is placed in the 

 focus of invisible rays, nothing is seen. 



10. When a solution of sulphate of quinine, or a piece of ura- 

 nium-glass, is placed in the focus, nothing is seen. 



11. When the retina of the human eye is placed at the focus, 

 in which metal plates are raised to incandescence, nothing- 

 is seen. 



The injury to my eyes, resulting from this experiment, was, I 

 believe, less than that produced by the night-labour which the 

 writing of this article has imposed upon me. 



Royal Institution, December 1864. 



IX. Note on the History of Energy. 

 By P. G. Tait, M.A * 



IN the December Number of the Philosophical Magazine, Dr. 

 Akin has called in question the statement that Newton, in 

 a Scholium to his Third Law of Motion, "completely enunciated 

 the Conservation of Energy in ordinary mechanics." He calls 

 attention to the circumstance that the words " in omni instrumen- 



* Communicated by the Author. 



