PART III.

ON THE PRODUCTION AND COMMUNICATION OFHEAT AND LIGHT, AND THE PHENOMENAOF COMBUSTION.

523. These subjects are so intimately associated in nume-rous practical questions, that some preliminary remarks are ne-cessary, before entering on the individual chapters.

524. The precise nature of Heat and Light is still unknown,though numerous facts attest the very intimate connection thatsubsists between them, electricity, and magnetism, and give areasonable presumption in hoping, that the progress of sciencemay one day reveal some new fact bearing on their mutual con-nection, eventually simplifying the innumerable details that havebeen ascertained by experiment, and giving man a still more ex-tended power over the material world.

525. The most intense heat and brilliant light hithertodeveloped by artificial means, and capable of being employedfor various experimental purposes, though not sufficiently per-fect for ordinary use, is that which is produced by a stream ofgalvanism proceeding (''in vacuo) from one piece of charcoal to an-other, and more or less nearly into contact, according to the powerof the galvanic battery employed. The general appearance re-presented by such an apparatus, is given in the accompanyingfigure. This light has been repeatedly tried in individual apart-