246 ELEMENTARY LESSONS ON [CHAP. iv. 



the celebrated Geissler, are employed. These are merely 

 tubes of thin glass blown into bulbous or spiral forms, 

 provided with two electrodes of platinum wire fused into 

 the glass, and sealed off after being partially exhausted 

 of air by a mercurial air-pump. Of these Geissler tubes 

 the most useful consist of two bulbs joined by a very 

 narrow tube, the luminous effects being usually more 

 intense in the contracted portion. Such tubes are 

 readily illuminated by a spark from an electrophorus or 

 electric machine ; but it is more common to work them 

 with the spark of an induction coil (Fig. 148). 



Through such tubes, before exhaustion, the spark passes 

 without any unusual phenomena being produced. As 

 the air is exhausted the sparks become less sharply 

 defined, and widen out to occupy the whole tube, 

 becoming pale in tint and nebulous in form. The 

 negative electrode exhibits a beautiful bluish or violet 

 glow, separated from the conductor by a narrow dark 

 interval, while at the positive electrode a single small 

 bright star of light is all that remains. Frequently the 

 light breaks up into a set of strice, or patches of light of 

 a cup-like form, which vibrate to and fro between darker 

 spaces. In nitrogen gas the violet aureole glowing 

 around the negative pole is very bright, the rest of the 

 light being rosy in tint. In oxygen the difference is not 

 so marked. In hydrogen gas the tint of the discharge 

 is bluish, except where the tube is narrow, where a 

 beautiful crimson may be seen. With carbonic acid gas 

 the light is remarkably white. Particles of metal are 

 torn off from the negative electrode, and projected from 

 its surface. The negative electrode is also usually the 

 hotter when made of similar dimensions to the positive 

 electrode. It is also observed that the light of these 

 discharges in vacuo is rich in those rays which produce 

 phosphorescence and fluorescence. Many beautiful 

 effects are therefore produced by blowing tubes in 

 uranium glass, which fluoresces with a fine green light, 



