[34] _Phil. Mag._, May and July, 1897. He also shows electrical cohesion by an emulsion of oil and water, the two liquids, thoroughly shaken up, at once separating when exposed to strong electrical influence. In so far as the approximate contact is not between _surfaces_, but between points consisting of relatively few molecules, the attractive pressure is greater rather than less. Thus to take an extreme case, the attraction between two oppositely-charged molecules differing only by a volt from each other, and separated by a thin film like the black spot of a soap-film whose thickness was so admirably measured by Profs. Reinold and Rücker, is over 1,000 atmospheres in intensity. These differences of potential across thin films cannot continue for any time, unless a battery is used, for the films do not really insulate; they are able however to act as dielectrics for an instant, and to be burst with what we must be allowed to call a spark, though an infinitesimally small one, if the momentary strain caused by the impulsive rush of electricity is too great. APPENDIX I. PROF. HUGHES’S OBSERVATIONS. An account of the history of the coherer principle would not be complete without a reference to an interesting reminiscence of early observations recently put on record by the discoverer of the microphone. At each stage of his observations of electrical cohesion between metals the author was confronted by a reference to some earlier observations of Prof. Hughes, and he felt sure that during the work on the microphone many or all of the phenomena he was then observing must have been previously encountered by Prof. Hughes. No full account was at that time available, however, but now it is clear that the observations were made (like some of Edison’s on what he called etheric force, and like the very remarkable still earlier ones of Joseph Henry) before the time of Hertz, when the existence of electric waves able to excite sparks or perform other energetic acts was unlooked for and not clearly understood. Nevertheless, at this early period it is clear that Prof. Hughes observed, though he did not follow up the observation, not only the occurrence of electric waves or impulses in space, but also the coherer method of detecting them; in fact, that he unwittingly made the earliest experiments on wireless telegraphy by this plan. The simplest way is to quote Prof. Hughes’s letter to Mr. J. J. Fahie from _The Electrician_, May 5, 1899, p. 40, beginning with Mr. Fahie’s letter as an introduction:-- _Extract from recent letter from Mr. J. J. Fahie to Prof. Hughes._