366 Profs. J. Dewar and J. A. Fleming. On the Dielectric 



two terms of the Cauchy's formula,* we find that for carbon bi- 

 sulphide the value of (/too) 2 is 2"01, and this is somewhat less than the 

 value (2'67) of K at ordinary temperatures. It is, however, much 

 nearer to the value (2 "24) which we find for K at the liquid air 

 temperature. Again, Dr. J. Hopkinson's value for the dielectric 

 constant of castor oil at normal temperature is 4*78, and the square 

 of the refractive index of this body for infinite waves is 2*153. Our 

 value for the dielectric constant of castor oil is 4/23 at 20° C. and 219 

 at -185° C. 



Hence, in the case of castor oil at the temperature of liquid air 

 there is practically a very fair agreement between the values of K 

 and O^a,) 2 for a low frequency of electromotive force reversals, whereas 

 at normal temperatures K has nearly twice the value of (/too) 2 . 



The same fact holds good for olive oil. At normal temperatures 

 its dielectric constant K is 3*16, and the square of its infinite refrac- 

 tive index {p^y is 2'131. At the temperature of liquid air we find 

 the dielectric constant of olive oil to be 2'18 for low frequency. 

 Hence, in these cases a large part of the difference between K and 

 O^oo) 2 which exists at normal temperature disappears at very 

 low temperatures, and castor oil and olive oil then make in this 

 respect even a better agreement than carbon bisulphide. One 

 other point is of interest. The temperature variation of dielectric 

 constant for both olive oil and carbon bisulphide appear to be nega- 

 tive at ordinary temperatures,"]" that is to say, their dielectric con- 

 stants decrease with rise of temperature. At low temperatures, 

 however, we find their constants to have a positive temperature 

 coefficient ; in other words, they increase with temperature. Hence, 

 it follows that, like ice and glycerine, they have at some temperature 

 a maximum value for their dielectric constant. 



The following table shows the relative values of the dielectric 

 constants (K) of certain of these organic bodies at 15° and at 

 — 185° C for low frequency. At the same time the value of (/too) 2 

 calculated by Cauchy's formula is placed against them as far as 

 we have been able to find it determined. 



It is clear, therefore, that in all the above cases a very low tempera- 

 ture extinguishes the high dielectric values many organic bodies have 

 in the liquid condition, and it produces the same effect, therefore, as 

 a sufficient increase in the frequency of the electromotive force re- 

 versals. Either of these operations tends to bring down the high 

 value of the dielectric constant to a value much more comparable in 

 magnitude with the square of the optical refractive index as calcu- 



'* We here make the assumption that even for a highly dispersive body like carbon 

 bisulphide, the Cauchy formula is applicable for calculating (m x ) 2 , an assumption 

 perhaps not warranted. 



t See Cassie, ' Roy Soc. Proc.,' vol. 46, 1889. 



