214 SCIENTIFIC RECREATIONS. 



is independent of the diameter of the balls or globes between which the 

 spark strikes." 



Many examples might be given of the spark discharge of the electric 

 current. This form is seen in the blue line extending between the knob of 

 a machine and the hand, and the duration of a spark with a jar charged 

 with an induction coil is stated by Professor Rood to vary, but the brightest 

 portion with a jar of 1 14 square inches only existed for the I75th billionth 

 of a second, and with a smaller surface was much shorter. Such a spark 

 may be conducted to a plate of gunpowder and will not ignite it, because 

 the time of the duration of the " fire " is not sufficiently long ; the powder 

 will be scattered, but not ignited. If, however, a partly non-conducting 

 medium be interposed between the jar and the powder, so that the spark be 

 retarded a little, the gunpowder will be fired. 



While speaking of electric discharge we may remark upon the beau- 

 tiful effect of lightning. These discharges are sometimes miles long, and by 

 the return stroke from the cloud may kill a person a long way from the 

 actual discharge. This phenomenon was illustrated by Viscount Mahon in 

 1779, in a very interesting book on the principles of electricity. 



There are different ways in which the electric discharge shows itself. 

 We have spoken about the spark discharge which, however, is found to pre- 

 sent 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 



