In one instance the tube is usually of the form shown on Plate X. fig. 9, supported on a base with a brass ball electrode at the lower end, and a pointed wire at the upper. In another case the tube is of the form shown on same Plate, fig. 8. After exhaustion it is permanently closed, the current passing through it by means of the platinum-wire electrodes introduced into each end of the tube. The first form of tube is usually excited by a frictional plate machine; the second by a galvanic current from a Grove or bichromate battery, which, by the aid of a Ruhmkorff coil, has had its character changed from quantity to intensity. In each instance, upon connexion with the source supply of the electric current, a very similar effect is produced. [Sidenote: Effects described.] Brilliant streams of rose-coloured light pass between the electrodes, sometimes as a single luminous misty band, sometimes in divided vibrating sprays or streams, and sometimes in a flaky column of striæ. All this, before the spectroscope took its part in the investigation, we were content to accept as a very fair and probable explanation of the Aurora accompanied by a mimic representation of the phenomenon. These appearances may, of course, be produced at will in tubes having electrodes; but it is, moreover, possible to produce them, though with less effect, in certain other forms of tube having no such direct communication with the external electric machine. One electrode only may be connected with the coil or electrical machine. The appearance is then a faint representation of what happens when the current entirely passes (but see experiments with a single wire detailed in Part III.). [Sidenote: Tube without electrodes.] In the case of an exhausted tube having no electrodes, the wires from the coil may be made into a little helix and placed at each end of the tube, and the induced currents within will show themselves in flashes and streams of light, varying in colour and tint according to the gaseous or other contents of the tube. [Sidenote: Tube excited by friction.] In some cases the ordinary forms of galvanic or electrical machine for supplying the current of electricity may be dispensed with. A long straight tube exhausted and closed at each end, and without electrodes, Plate X. fig. 6, being slightly warmed and then excited by friction with the dry hand or a piece of flannel, silk handkerchief, or the like, is soon filled with the most brilliant flashes of light playing in the interior, and when once thoroughly charged needs but little further excitation to keep up the effect. [Sidenote: Geissler’s mercury tube.]