EXPERIMENTS. 2 I 5 



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. 



Professor Tyndall at the Royal Institution showed a very pretty experi- 

 ment. He took a funnel with a very fine bore, and permitted sand to flow 

 from it as it will in the hour-glass. When he permitted the electric current 

 to come in contact with the sand, however, it, instead of falling vertically 

 to the table, spread out fan-like, each grain repelling and being repelled by 

 its neighbour with an effect very beautiful to see. Luminous effects have 

 frequently been produced by passing an electric spark through various bodies. 

 For instance, a lump of sugar can be made quite brilliant in the dark by 

 passing electricity through it, and there are other substances similarly effected. 

 Even eggs and some fruits are thus made phosphorescent. The illumination 

 of the " diamond " covered Leyden jar is familiar to all who ever attended a 

 lecture on Electricity. 



The various effects of the electric discharge need not here be described. 

 We have witnessed the results of lightning, but even in our laboratory many 

 pretty little experiments can be made, such as the perforation of a card by 

 the electric discharge. The chemical effects are various. Decomposition of 

 water is effected by electricity, and the discharge can also be, and has been 

 utilized for military purposes, such as employed by Professor Abel in his 

 fuse, and in his apparatus for firing mines. Experiments at Chatham and 

 elsewhere have been very successful in the ap- 

 plication of electricity to modern warfare. 



We will illustrate one or two of these. A 

 thick card should be placed, as in the illustra- 

 tion (fig. 2 1 6), between two insulated points, and 

 to the lower portion of the apparatus a chain 

 be attached, held in the hand, and wound round 

 the Leyden jar. If then the knob of the jar 

 and the knob above the upper point be brought 

 together, the spark will pass through the card. 



In the same manner a glass may be per- 

 forated if the current be stronger. Of course 

 the whole apparatus and particularly the plate 



must be quite dry, and it will be better to put a drop of oil under the 

 upper needle point so as to prevent the electricity spreading over the glass. 



