Although the phenomenon of the production of light by all solid bodies, when their temperature is raised to a certain degree, is one of the most familiar, no person so far as I know has hitherto attempted a critical investigation of it. The difficulties environing the inquiry are so great that even among the most eminent philosophers a diversity of opinion has prevailed respecting some of the leading facts. Thus Sir Isaac Newton fixed the temperature at which bodies become self-luminous as 635°; Sir Humphrey Davy at 812°; Mr. Wedgwood at 947°; and Mr. Daniel at 980°. As respects the nature of the light emitted, there are similar contradictions. In some philosophical works of considerable repute it is stated that when a solid begins to shine it first emits red and then white rays; in others it is asserted that a mixture of blue and red light is the first that appears.

I have succeeded in escaping or overcoming many of the difficulties of this problem, and have arrived at satisfactory solutions of the main points; and as the experiments now to be described lead to some striking and perhaps unexpected analogies between light and heat, they commend themselves to our attention, as having a bearing on the question of the identity of those principles.