260 Dr. J. Kerr's Electro-optic 



Concluding Remarks. 



30. Bodies under electric stress are bircfringeni, with refer- 

 ence to line of electric force as axis. — This property has now 

 been exemplified in such a number and such a variety of 

 dielectrics, that it ought to be admitted as a general property 

 of matter. There do occur cases (in the formic ethers &c.) 

 in which the purity of electro-optic double refraction is lost : 

 but these are no true exceptions to the general statement ; 

 they are merely cases of disturbance, cases in which the 

 regular effect is masked, wholly or partly, by the presence 

 of other effects. Gross movements and heat appear to be 

 the principal causes of disturbance. 



Dielectrics are divisible into two classes, the positive and the 

 negative, corresponding to the two so-named classes of uniaxal 

 crystals. — This also has been abundantly confirmed by experi- 

 ment, not as a rough adaptation of the notions and terms of 

 Optics to our present subject, but as an exact physical truth. 

 Except in cases of impure action or of mixed optical effects, 

 cases Avhich are comparatively rare in fact, the experimental 

 contrast between the two kinds of dielectric is always as pure 

 to sense, and under any tests yet applied, as is the contrast 

 between quartz and Iceland spar. The two finest examples 

 of the positive class are carbon disulphide and amyl chloride ; 

 and these are far superior in power to any negative dielectric 

 yet discovered. As interesting examples of the negative class, I 

 select the following four, arranging them (according to my 

 samples) in the order of descending powers : — amyl oxide, 

 chloroform, capryl alcohol, aniline. 



Among bodies yet examined, the positive dielectrics are more 

 numerous than the negative. — Of 13 amyl compounds, 10 are 

 purely positive, and 3 purely negative; and similarly in other 

 cases. Br, P, S, the only elements yet examined, are all 

 purely positive. 



The electro-optic and chemical characters of compounds are 

 closely connected. — The hydrocarbons are positive, the fixed 

 oils and fats negative, the fatty acids positive, the alcohols 

 negative, &c. 



The electro-optic character of a chemical compound is not de- 

 termined by its empirical formula. — Isomeric bodies are some- 

 times electro-optically similar, and sometimes not. This is 

 shown in the four following sets of isomers or polymers : 

 propionic acid, methyl acetate, ethyl formate — all positive ; 

 butyl alcohol and ethyl oxide, both negative ; aldelryde, 

 butyric acid, ethyl acetate — the first negative and the other 

 two positive ; allyl alcohol, acetone, caproic acid, methyl 

 valerate— the first negative and the others positive. 



