Experiments on various Liquids. 261 



Every chemically homologous series of bodies yet examined 

 exhibits a certain constancy of electro-optic character. — Thus_, 

 the oxides, hydroxides, and sulphides of the alcohol radicals 

 are negative ; while the hydrosulphides, the iodides, bromides, 

 chlorides, butyrates, benzoates, &c. are positive. 



Through each of these homologous series, there is generally a 

 progression of electro-optic power, the higher members being the 

 stronger. — The common alcohols and the fatty acids (liquid at 

 ordinary temperatures) afford very good illustrations. The 

 two series of bodies are negative and positive respectively; 

 and in each of the series, from member to higher member, there 

 is a regular increase of absolute electro-optic power. The pro- 

 gression is not maintained in the higher parts of the acid 

 series, the palmitic and stearic acids being (as the other fats 

 are) strongly negative. 



This progression, in singular cases, takes the form even of a 

 change of sign. — The only examples yet known are afforded 

 by the two last mentioned series, which begin thus : 

 methyl alcohol + 1 f — formic acid 



ethyl alcohol — J \ + acetic acid 



This occurrence of contrary changes of sign at corresponding 

 points of the two series is surely a very remarkable fact. 

 The two following inferences are perhaps worthy of notice. 



Oxygen is probably a negative dielectric. — Putting n = in 

 the general formulas of alcohol and acid, we obtain H 2 and 

 2 as the starting-points of the two series. Water and oxygen 

 should therefore have the same signs as methyl alcohol and 

 formic acid respectively. As far as water is concerned, the in- 

 ference is confirmed already by experiment. 



Our terms " positive'' 'and "negative," borrowed originally from 

 Physical Optics, appear now to be terms intrinsically appropriate 

 to the subject. — For in the acids we see a regular increase of 

 positive power showing itself at one point of the series as a 

 passage from — to + , and, again, in the alcohols a regular 

 increase of negative power showing itself at one point as a 

 passage from + to — . 



31. Electro-optic double refraction is brought about by a 

 special state of the dielectric, a state of essentially directional 

 strain, which is a concomitant and a condition of the main- 

 tenance of electric stress. This appears to me to be a principle 

 that must underlie any admissible explanation of the phe- 

 nomena. The carefully grounded and very valuable concep- 

 tions of Faraday and Maxwell point in this one direction ; 

 and the simple facts of the case are, of themselves, cmite deci- 

 sive. What clearer manifestations of directional strain could be 



Phil Mag. S. 5. Vol. 13. No. 81. April 1882. Y 



