station till the effect was compensated by proportionany altering
the distances of the heated cylinders from the halls. In some further
experiments, instead of blackening one of the flat surfaces of one of
the cylinders, the other coverings used in the foregoing trials 'were
applied, and the results Were such as might have been expected.
They all tended to prove that different bodies, or rather difl’erent
surfaces, emit heat not by any conducting power in themselves, or
in the surrounding bodies, but by a power which is here called ra-
diation, the nature of which had hitherto escaped our notice.
Several experiments were next made with heated cylinders of dif- ferent metals, but the results proved that all metals give off heat with the same facility, or rather with the same celerity. May not this, it is asked, be owing to their being all equally wanting in transparency 3 And does not this afford us a strong presumption that heat is in all cases excited and communicated by means of radiations, or as they may more properly be called undulations ?
Before these questions can be solved, another and a very important point in this inquiry must be decided, viz. whether bodies are cooled in consequence of the rays they emit, or by those they receive? Our author was manifestly led to this problem by the celebrated experi- ment of Prof. Pictet, from which it appears that rays or emanations which (like light) may be concentrated by concave mirrors, proceed from cold bodies; and that these rays when so concentrated, are ca- pable of affecting an air thermometer in a manner perfectly percep- tible. The first experiment on this subject was to ascertain the ex- istence of these cold emanations universally; and this being success- fully efi'ected, it is proved by other processes, the detail of which would far exceed our bounds, that the radiation of cold as well as of hot bodies being established, the rays which proceed from cold bodies have likewise the power of generating cold<in warmer bodies which are exposed to their influence.
The object of another set of experiments was to ascertain whether all cold bodies at the same temperature emit the same quantity of rays; or whether (as is the case with respect to the calorific rays emitted by hot bodies,) some substances emit more of them than others. Here it was a great gratification to the author to find in the first experiment that the frigorific rays, from a blackened metallic surface, were much more powerful in generating cold than those which proceeded from a similar metallic surface of exactly the same temperature, but without any coating.
Observing that the approach of the hand to one of the balls of the thermoscope affected the indications very sensibly and rapidly, it oc- curred that perhaps animal substances emit both calorific and frigo- rific rays more copiously than other substances, and that probably living animal bodies emit them in still greater abundance than dead animal matter. This was confirmed by a very conclusive experiment, in‘which one of the metallic surfaces was covered with goldbeater’s skin, and which surface emitted at least twenty-five times more ca- lorific rays than a naked surface. The frigorific rays from the animal
