There are different ways in which the electric discharge shows itself. We have spoken about the spark discharge which, however, is found to present very different appearances in varying conditions. Professor Faraday proved that the colour of the electric sparks showed in air, when obtained with brass balls, the intense light and blue colour so familiar to all. In nitrogen they are even bluer. In oxygen again the sparks are much _brighter_ than in air, but not so _brilliant_. In hydrogen they become crimson, but the sound is almost inaudible because of the physical character of the gas. In carbonic acid gas they are almost the same as in atmospheric air, only more irregular. In dry muriatic gas they are nearly white and very bright. In coal gas the colours vary—sometimes being green and sometimes red. Occasionally the same spark will be red and green at different extremities, and even _black_ portions have been observed. The density and pressure of the atmosphere has been proved to exercise considerable influence upon the spark discharge. The “Brush” discharge is shown in “a series of intermittent discharges which appear continuous.” This discharge assumes the shape of a fan. “It is accompanied with a low chattering sound,” which is the result of the separate and continuous discharges, and Faraday also demonstrated that its effects varied according to the medium in which it was exhibited. The effect of the air pressure on electricity may be observed in the following way:— If we pass a spark through rarefied air by an apparatus known as the Electric Egg, we may obtain many curious effects. The “egg” consists of a glass globe, through which enter two rods with a knob upon each inside end. The upper rod is moveable, and held in its place by a “cap” like the lower rod. There is a stop-cock in the lower cap, so that the egg may be fastened to any plate or stand. When the egg is filled with air, the electric spark passed into the glass globe has the usual appearance, but as the air is gradually rarefied by an air-pump, the spark assumes beautiful forms and colours. As the exhaustion continues, however, we shall find the spark decreasing in brilliancy, and finally the spark will cease to be visible. It thus is shown that the colour of the spark depends upon the gaseous medium and on the material of the conductors, and when the electric spark is faint this medium can be observed, for nitrogen will produce a blue tinge and carbonic acid a green; hydrogen gives us a red, as already remarked. By multiplying the number of eggs and plates of glass, and placing discs of tinfoil in various shapes at certain distances, many beautiful figures may be observed when the spark is set free.