458 THE POPULAR SCIENCE MONTHLY.
revealed the cause of this beautiful phenomenon. Wells knew that through the body of our atmosphere invisible aqueous vapor is every- where diffused. He proved that grasses and other bodies on which dew was deposited were powerful emitters of radiant heat ; that, when nothing existed in the air to stop their radiation, they became self- chilled ; and that while thus chilled they condensed into dew the aqueous vapor of the air around them. I do not suppose that any theory of importance ever escaped the ordeal of assault on its first enunciation. The theory of Wells was thus assailed ; but it has proved immovable, and will doubtless continue so to the end of time.
The interaction of scientific workers causes the growth of science to resemble that of an organism. From Faraday's tiny magneto- electric spark, shown in this theatre half a century ago, has sprung the enormous practical development of electricity at the present time. Thomas Seebeck in 1822 discovered thermo-electricity, and eight years subsequently bars of bismuth and antimony were first soldered together by Nobili so as to form a thermo-electric pile. In the self-same year Melloni perfected the instrument and proved its applicability to the investigation of radiant heat. The instrumental appliances of science have been well described as extensions of the senses of man. Thus the invention of the thermopile vastly augmented our powers over the phenomena of radiation. Melloni added immensely to our knowledge of the transmission of radiant heat through liquids and solids. His results appeared at first so novel and unexpected that they excited skepticism. He waited long in vain for a favorable report from the Academicians of Paris ; and finally, in despair of obtaining it, he pub- lished his results in the " Annales de Chimie." Here they came to the knowledge of Faraday, who, struck by their originality, brought them under the notice of the Royal Society, and obtained for Melloni the Rumford medal. The medal was accompanied by a sum of money from the Rumford fund ; and this, at the time, was of the utmost im- portance to the young political exile, reduced as he was to penury in Paris. From that time until his death, Melloni was ranked as the foremost investigator in the domain of radiant heat.
As regards the philosophy of the thermopile, and its relation to the great doctrine of the conservation of energy, now everywhere accepted, a step of singular significance was taken by Peltier in 1834. Up to that time it had been taken for granted that the action of an electric current upon a conductor through which it passed was always to generate heat. Peltier, however, proved that, under certain circum- stances, the electric current generated cold. He soldered together a bar of antimony and a bar of bismuth, end to end, thus forming of the two metals one continuous bar. Sending a current through this bar, he found that when it passed from antimony to bismuth across the junction, heat was always there developed, whereas, when the direction of the current was from bismuth to antimony, there was a