[24] The "earth" itself probably only conducts electrolytically. All such materials as sand, clay, chalk, etc., and most surface soils are fairly good insulators when very dry, but conduct in virtue of moisture present in them.
[25] The Electrician, Vol. XL., p. 86 (leader).
[26] British Patent Specification, C. and S. A. Varley, No. 165, 1866.
[27] See also Journal de Physique, Vol. V., p. 573, 1886.
[28] See Comptes Rendus, Vol. CXI., p. 785; Vol. CXII., p. 112, 1891; or La Lumière Electrique, Vol. XL., pp. 301, 506, 1891; or The Electrician, Vol. XXVII., 1891, pp. 221, 448.
[29] See The Electrician, Vol. XXIX., 1892, pp. 397 and 432.
[30] Mr. W. B. Croft, Proc. Phys. Soc., Vol. XII., p. 421. Report of meeting on October 27, 1893.
[31] See Professor Minchin, Proc. Phys. Soc., November 24, 1893; or The Electrician, Vol. XXXII., 1893, p. 123. See also Professor Minchin, [Phil Mag.], January, 1894, Vol. XXXVII., p. 90, "On the Action of Electromagnetic Radiation on Films containing Metallic Powders."
[32] This lecture was afterwards published as a book, the first edition bearing the same title as the lecture—viz., "The Work of Hertz and Some of His Successors." In the second edition, published in 1898, an appendix was added (p. 59) containing "The History of the Coherer Principle," and the original title of the work had prefixed to it "Signalling Without Wires."
[33] See The Electrician, Vol. XXVII., p. 222, 1891. E. Branly, "Variations of Conductivity under Electrical Influence."