Conservation of Energy 15 



the structural arrangement and motion of the 

 elements which form a piece of iron wire is such, 

 that they can act as a means of transmuting 

 magnetic energy into electrical or into mechani- 

 cal energy. A thread of platinum is so sensitive 

 that it reacts by a variation of electric conduc- 

 tivity when struck by a ray of light of so feeble 

 an intensity, that it can produce an elevation of 

 temperature amounting to only one hundred- 

 millionth of a degree. Hertzian waves which 

 have travelled over hundreds of miles, and are 

 therefore extremely feeble, nevertheless so 

 modify the structure of the metal they reach 

 as to produce marked changes in its electric 

 conductivity. Again, it has been shown that 

 metals become for a time less sensitive after 

 constant excitation, but regain their irritability 

 after an interval of repose ; while their peculiar 

 qualities may be excited or depressed or even 

 abolished by chemical substances. 1 



The structural arrangement of the molecules 

 which constitute the basic substance of living 

 protoplasm makes it peculiarly sensitive to 



1 Annual Report of the Smithsonian Institution for 1903, p. 288. 

 Also Human Speech, by N. C. Macnamara, p. 13. 



