+----------------------------------+-------+--------------------+
| Substance. | K. | Authority. |
+----------------------------------+-------+--------------------+
| Glass, double extra dense flint, | | |
| density 4.5 | 9.896 | J. Hopkinson |
| Glass, light flint, density 3.2 | 6.72 | " |
| Glass, hard crown, density 2.485 | 6.61 | " |
| / | 2.24 | M. Faraday |
| | | 2.88 | Coullner |
| Sulphur . . . . . . . . . . .< | 3.84 | L. Boltzmann |
| | | 4.0 | P.J. Curie |
| \ | 2.94 | P.R. Blondlot |
| / | 2.05 | Rosetti |
| | | 3.15 | Boltzmann |
| Ebonite . . . . . . . . . . .< | 2.21 | Schiller |
| \ | 2.86 | Elsas |
| India-rubber, pure brown | 2.12 | Schiller |
| India-rubber, vulcanized, grey | 2.69 | " |
| Gutta-percha | 2.462 | J.E. H. Gordon |
| / | 1.977 | Gibson and Barclay |
| | | 2.32 | Boltzmann |
| Paraffin . . . . . . . . . . .< | 2.29 | J. Hopkinson |
| \ | 1.99 | Gordon |
| / | 2.95 | Wallner |
| Shellac . . . . . . . . . . .< | 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 deg. |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.