THE STORY OF NINETEENTH-CENTURY SCIENCE 



IV 



The gradual permeation of the field by the great 

 doctrine of conservation simply repeated the history 

 of the introduction of every novel and revolutionary 

 thought. Necessarily the elder generation, to whom 

 all forms of energy were imponderable fluids, must pass 

 away before the new conception could claim the field. 

 Even the word energy, though Young had introduced 

 it in 1807, did not come into general use till some time 

 after the middle of the century. To the generality of 

 philosophers (the word physicist was even less in favor 

 at this time) the various forms of energy were still 

 subtle fluids, and never was idea relinquished with 

 greater unwillingness than this. The experiments of 

 Young and Fresnel had convinced a large number of 

 philosophers that light is a vibration and not a sub- 

 stance; but so great an authority as Biot clung to the 

 old emission idea to the end of his life, in 1862, and held 

 a following. 



Meantime, however, the company of brilliant young 

 men who had just served their apprenticeship when the 

 doctrine of conservation came upon the scene had grown 

 into authoritative positions, arid were battling actively 

 for the new ideas. Confirmatory evidence that energy 

 is a molecular motion and not an " imponderable " form 

 of matter accumulated day by day. The experiments of 

 two Frenchmen, Hippolyte L. Fizeau and Leon Foucault, 

 served finally to convince the last lingering sceptics that 

 light is an undulation ; and by implication brought heat 

 into the same category, since James David Forbes, the 

 Scotch physicist, had shown in 1837 that radiant heat 

 conforms to the same laws of polarization and double 



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