This paper is divided into three sections, each containing a distinct investigation.

The First Section is on the Variable Polarizability of different kinds of Heat. This fact, namely, that when polarization takes place by refraction through a bundle of parallel mica plates, some kinds of heat are less completely polarized than others at the same incidence, the author proposed to establish in the second series of his researches. His investigations, however, having been considered by some persons as open to objection, and directly contrary results having been obtained by M. Melloni in his repetition of these experiments, the author was induced to investigate the subject anew and with every precaution.