16 Prof. A. Kundt on the Indices 



Connexion between the Indices of Refraction of the Metals and 

 their Power of Conducting Light and Electricity. 



If we glance at the values of n (compared on p. 12), the 

 connexion between the indices of refraction and the con- 

 ducting power of the metals for light and electricity at once 

 strikes the eye. Those which have the smallest indices, and 

 for which consequently the velocity of light is the greatest, 

 are also the best conductors of heat and electricity. 



The galvanic specific conductivity and the coefficient of 

 conduction for heat of a substance are perfectly definite values 

 for a given temperature ; the velocity of light in a body at a 

 given temperature is not, for it varies with the wave-length. 

 If therefore we wish to further investigate and formulate the 

 above suggested universal connexion, we shall have to settle 

 what is to be understood by the velocity of light. The for- 

 mulse of dispersion for transparent bodies lead always to a 

 definite limiting value of index of refraction with increasing 

 wave-length. That such a limit exists also for the metals 

 cannot indeed be proved, but is probable. For our determi- 

 nations those values are probably the nearest which were 

 obtained for red light. 



If we take the velocity of red light in silver as 100, then, 

 according to the table on p. 12, we obtain the following 

 numbers for the other metals : — 



Silver 100 



Gold 71 



Copper 60 



Platinum 15'3 



Iron 14-9 



Nickel 12-4 



Bismuth 10-3 



A glance at the above table shows that with one exception, 

 viz. bismuth, the values of v fall within the limits of the num- 

 bers for the electric conductivity, obtained by the different 

 observers, when that of silver is taken as 100*. The number 

 60 for copper appears somewhat too small. It is, however, to 

 be observed that the electrolytic copper of our double prism 

 might possibly contain some cuprous oxide, which would make 

 the index of refraction too great, and the velocity of light too 

 small. The conductivity of bismuth is given by all observers 

 considerably less than 10. The bismuth with which the de- 

 terminations of conductivity were made was, however, always 



* Wiedemann, Lehre von der Electricitat, 3rd edit. vol. i. p. 503. 



