Geissler has introduced a form of tube in which electricity in its form of flashes and glow of light is produced by the friction of mercury. The outer tube is strong, and contains within it a smaller tube of uranium glass with balls blown upon it (Plate X. fig. 7). The tubes are exhausted and a small quantity of mercury is introduced which has access to both surfaces of the inner tube, as well as to the inner surface of the outer tube. Upon the tube being reversed end for end or shaken, the mercury runs up or down the tube and causes a very considerable display of whitish light. The before-described tubes are also referred to, and their spectra described, in the section “On the comparison of some tube and other Spectra with the Aurora” (Part II.). The aura or brush from the electrical machine has been considered as resembling the Aurora, while the hissing and crackling accompanying it has been supposed to corroborate the reports of similar noises having been heard during an auroral display. [Sidenote: Prof. Lemström’s instrument to demonstrate the nature of Auroræ.] Prof. Lemström, of the University of Helsingfors, has devised an instrument for the purpose of demonstrating that Auroræ are produced by electrical currents passing through the atmosphere. An illustration of this instrument (for which I am indebted to the Editor of ‘Nature’) is introduced (fig. 1). The instrument was exhibited at the recent Scientific Loan Collection at South Kensington, and a full description of it, together with an essay by Prof. Lemström, “On the Theory of the Polar Light,” will be found in the third edition of the Official Catalogue, p. 386. no. 1751. The apparatus is intended to show that an electric current passing from an insulated body does not produce light in air of normal pressure; but as it rises to the rarefied air in the Geissler tubes a phenomenon very like the real Polar Light is produced. [Illustration: Fig. 1.] A is an electrical machine, the negative pole being connected with a copper sphere and the positive with the earth. _s s´_ is of ebonite as well as R R _d_, so that B is quite insulated as the earth is in space. B is surrounded by the atmosphere. _a´ a´ a´ a´ a´ a´_ are a series of Geissler tubes with copper ends above and below. All the upper ends are connected with a wire which goes to the earth; consequently a current runs in the direction of the arrows through the air, and the Geissler tubes become luminous when the electrical machine is set into operation.