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 



