ANISOTROPIC MEDIA. 617 



that is to say, that which corresponds to the greatest wave length. 

 For paraffine, for instance, the refractive indices of the extreme 

 luminous rays vary from 1.43 to 1.45, and the best experiments 

 show that the specific inductive power is equal to 2.29, the square 

 root of which 1.51 is not greatly different from the refractive index. 

 The agreement is much less satisfactory with most of the trans- 

 parent solid dielectrics, such as the different kinds of glass, Iceland 

 spar, fluor spar, and quartz ; their specific inductive capacity is always 

 higher, and is sometimes twice the square of the refractive index. 

 This is also the case with the animal and vegetable oils, according 

 to the recent experiments of Dr. Hopkinson. 



For gases, in which the refraction is less, and the dispersion 

 may be neglected, the refractive power n 2 - i is proportional to the 

 specific gravity, or to the pressure, if the temperature is constant ; 

 it ought to follow from this that the specific inductive capacity 

 also increases in proportion to the pressure, and by the same co- 

 efficient as the refractive power. This conclusion seems to have 

 been verified by the researches of M. Boltzmann. 



The experimental control cannot therefore be considered suffi- 

 cient to confirm the theory; but too much importance must not 

 be attached to this apparent disagreement, if we take into account 

 the fact that the specific inductive capacity diminishes with the 

 duration of the electrification. But the period of electrical oscil- 

 lations, which must be assumed in order to explain luminous 

 phenomena, is out of all proportion with the shortest interval 

 of time that can be realised in electrical experiments. 



In any case, this correlation between the electrical and optical 

 properties of a medium may be considered as, at all events, a first 

 approximation to a theory which remains to be more minutely 

 developed. 



635. ANISOTROPIC MEDIA. In order to extend the theory to 

 anisotropic media, we should, in strictness, know the relation between 

 the molecular constitution of a medium and its electrical properties ; 

 but without making any hypothetic supposition, it is sufficient if 

 we assume that the specific inductive capacity of the medium is 

 not the same in different directions ; in other words, that the 

 electromotive force, instead of being proportional to the displace- 

 ment, and in the same direction, is connected with the displacement 

 by a system of linear equations, as for the phenomena of thermal 

 expansion. 



In this case, there are three rectangular directions, along which 

 the electromotive force is in the direction of the displacement ; 



