1883.] on Thoughts on HadiatioUj Theoretical and Practical. 255 



their radiation, they became self-eliilled ; and that while thus chilled 

 they condensed into dew the aqueous vapour 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 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 applica- 

 bility to the investigation of radiant heat. The instrumental appli- 

 ances 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 scepticism. He waited long in vain for a favourable 

 Eeport from the Academicians of Paris ; and finally, in despair of 

 obtaining it, he published 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 Eoyal Society, and 

 obtained for Melloni the Rumford medal. The medal was accom- 

 panied by a sum of money from the Rumford fund ; and this, at the time, 

 was of the utmost importance 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 development 

 of cold. By placing a drop of chilled water upon the junction of the 

 two metals, Lenz subsequently congealed the water to ice by the 

 passage of the current. 



The source of power in the thermopile is here revealed, and a 

 relation of the utmost importance is established between heat and 



Vol. X. (No. 76.) s 



