638 MECHANICAL EFFECTS OF THE DISCHARGE. 



bent into a ring, a chain of sufficient length is hung which is con- 

 nected with the outer coating of the jar. 



The passage of electricity through rarefied air may be very con- 

 veniently observed in Geisslers tubes. These are tubes of glass 

 filled with air or different gases in a very rarefied condition ; they 

 are hermetically closed and therefore always ready for use. At the 

 ends of the tube two platinum wires are fused air-tight into the 

 glass. The tubes are usually not simply straight, but are bent into 

 various pleasing forms ; they often contain bulbous enlargements, 

 either globular, oval, or of other shapes, the variety of form being 

 calculated to increase the brilliancy of the electrical phenomena 

 which they are used to exhibit. Frequently they are put together 

 of various kinds of glass, each of which appears of a different colour 

 when it transmits the light of an electrical discharge. Fig. IV of 

 the coloured frontispiece shows a tube of this kind, which contains 

 a small cup with a hollow stem of Uranium glass. This glass appears 

 in daylight of a yellowish-green colour, while by the light of a lamp it 

 is nearly quite yellow ; when seen by the light of the electric spark 

 its colonr is a splendid pure green. The outer ends of the platinum 

 wires are bent into small loops. A chain for connecting it with 

 the outer coating of the jar is attached to one of the loops, and the 

 knob of the charged jar is brought near the other loop quickly, but 

 cautiously, so as not to strike against the thin- walled tube and 

 break it. 



The tubes must be clamped very gently and cautiously in the 

 retort-stand ; they are in less danger of being broken if a small 

 wooden stand is used, shaped like a candlestick, into the cavity of 

 which one end of the tube may be stuck. The brilliant coloured 

 appearances which may be produced by means of these tubes 

 cannot be adequately represented by an illustration. They are 

 best observed during the evening in a room in which there is no 

 other light but that of a very small lamp, just sufficient to see the 

 apparatus which is used. 



The mechanical effects of the discharge upon solid 

 bodies may be most readily seen by placing a sheet of 

 paper upon the external coating of a charged jar. 

 Touching the middle of the paper with one knob of the 

 discharger, and bringing the other knob quickly near to 

 the knob of the jar, the paper will exhibit one or several 

 perforations where it was in contact with the jar. 



