r 134 i 



IX. Specific Inductive Capacities of Water, Alcohol, Sfc. 

 By J. Hopkinson*. 



IN the December number of the Philosophical Magazine 

 is a short article by Prof. R. A. Fessenden on the 

 Measurement of the Specific Inductive Capacities of Water, 

 Alcohol. &c. In this article he states that the high values 

 found by Cohn and Arons and others for water, alcohol, &c. 

 are not correct, but that the true values are in every one of 

 these substances very nearly equal to that called for by Max- 

 well's theory. He also states (hat all the determinations of 

 such substances as sulphur, &c. are incorrect. The observa- 

 tions said to be incorrect are not those of a single observer, 

 but of many whose results are in good accord. 



Taking solids first, it has been shown that the capacity of 

 light flint-glass at ordinary temperatures is the same whether 

 the time of charge is t ] second, or J second, that it is 

 independent of the potential of the charge, and that its value 

 is about 6*7|. It is also well known that this glass insulates 

 so well that it will hold its charge for months J. Is it sug- 

 gested that these results are vitiated by electrolytic polari- 

 zation or by a hypothetical laminated structure of the material? 

 The specific inductive capacity of ice has been determined by 

 Bonty§. He finds 78, practically the same value as Cohn 

 and Arons find for water. But the resistance is from 10 5 to 

 10 6 times as great as that of water, which quite precludes the 

 suggestion of electrolytic polarization. 



Turning to liquids, we have a large number of determina- 

 tions which also deviate from Maxwell's law. Take, for 

 example, castor-oil and ether. Both insulate well enough to 

 make ceriain that electrolytic polarization does not affect the 

 result. Quincke determined the capacity of ether || by three 

 very different methods and found it in each case about 4*7 ; 

 my result by a fourth method was 4*75. For castor-oil I 

 obtained by two very different methods 4*78 and 4"82^[. All 

 these results deviate much from Maxwell's law making use of 



* Communicated by the Author. 



t Phil. Trans, vol. clxxii. p. 372, and Gray's ' Absolute Measurement 

 in Electricity and Magnetism,' vol. i. p. 473. 

 X Phil. Trans, vol. clxvii. p. 610. 

 § Cumjjtes Bendus, p. 533, March 7, 1892. 

 || Gray, p. 483. 

 f Phil. Trans, (loc. cit.), and Proc. Roy. Soc. October 1887. 



