The first part of this paper describes the following groups of experiments:—

I. On the effect which roughening the surface of a body produces upon its radiation.

II. On the nature of that heat which is radiated by rock-salt at at 212° F.

III. On the radiation of glass and mica at high temperatures.

The second or theoretical portion of the paper has reference to the law which connects the radiation of a particle with its temperature, and to Dulong and Petit's experiments on this subject. The instruments used, and method of using them, were almost the same as described in the first series of these researches.