. . .< | 2.74 | Gordon | | \ | 3.04 | A.A. Winkelmann | | / | 6.64 | I. Klemencic | | | | 8.00 | P.J. Curie | | Mica . . . . . . . . . . . . .< | 7.98 | E.M.L. Bouty | | \ | 5.97 | Elsas | | Quartz-- | | | | along optic axis | 4.55 | P.J. Curie | | perp. to optic axis | 4.49 | P.J. Curie | | Ice at -23° |78.0 | Bouty | +----------------------------------+-------+--------------------+ Since Faraday's time, by improved methods, but depending essentially upon the same principles, an enormous number of determinations of the dielectric constants of various insulators, solid, liquid and gaseous, have been made (see tables I., II., III. and IV.). There are very considerable differences between the values assigned by different observers, sometimes no doubt due to differences in method, but in most cases unquestionably depending on variations in the quality of the specimens examined. The value of the dielectric constant is greatly affected by the temperature and the frequency of the applied electric force. TABLE II.--_Dielectric Constant (K) of Liquids._ +------------------------+--------+--------------+ | Liquid. | K. | Authority. | +------------------------+--------+--------------+ | Water at 17° C. | 80.88 | F. Heerwagen | | " " 25° C. | 75.7 | E.B. Rosa | | " " 25.3° C. | 78.87 | Franke | | Olive oil | 3.16 | Hopkinson | | Castor oil | 4.78 | " | | Turpentine | 2.15 | P.A. Silow | | " | 2.23 | Hopkinson | | Petroleum | 2.072 | Silow | | " | 2.07 | Hopkinson | | Ethyl alcohol at 25° C.| 25.7 | Rosa | | Ethyl ether | 4.57 | Doule | | " " | 4.8 | Bouty | | Acetic acid | 9.7 | Franke | +------------------------+--------+--------------+ TABLE III.--_Dielectric Constant of some Bodies at a very low Temperature (-185° C.) (Fleming and Dewar)._