H. V. Gill — Electric Discharge in Geissler Tubes. 413 



shows the result of this experiment. It will be noticed that 

 these heaps are farther apart than when the gas was at atmos- 

 pheric pressure, but the distance between them is more or less 

 constant. This result is satisfactory in showing that the air 

 waves even in so rare a medium possess considerable force. 



The apparatus was now arranged exactly as in the case of a 

 Geissler tube, Fig. 7. After a few sparks the powder was as 

 before thrown into heaps. Fig. 3, Plate IV, exhibits a result 

 for a pressure of 5 or 6 mm . These heaps are far apart, which 

 shows the violence of the spark and the great velocity of the 

 waves. This pressure was not sufficiently low to show strata; 

 but it was found that a Geissler tube, when substituted for the 

 tubes used in the last two cases, showed good stratification when 

 the spark passed through it simultaneously with the spark at C. 



When an induction coil was used the discharge was not suf- 

 ficiently strong to produce so marked an effect, but at each 

 spark a slight movement in the powder was observed ; and 

 when fine powder was used rings were sometimes formed on 

 the sides of the tube. 



These experiments prove that when the pressure is not too 

 low and when the spark is of sufficient strength (when the 

 spark is very intense the powder is projected in a heap mid- 

 way between the two poles), that there exist air waves capable 

 of producing the Kundt's tube effect. From this we may con- 

 clude at once that in the case of a Geissler tube these waves 

 likewise exist. It would appear to be conclusively proved that 

 the strata and the formation of the heaps of powder are due 

 to the same cause were we to obtain the Kundt tube effect 

 simultaneously with the stratified discharge, but this was 

 impossible for the following reasons. As we have seen by 

 experiments already cited, and from our own observation, if 

 the discharge is strong its great brightness makes it impossible 

 to observe any stratification ; while a strong discharge is essen- 

 tial to produce the heaps of powder. Thus though it is clear 

 that the nature of the spark is the same in a feeble discharge as 

 in an intense one, yet the accidental conditions are such that 

 the discharge favorable for producing the strata is unfavorable 

 for producing the Kundt effect, and vice versa. But if with 

 a strong discharge in a tube containing a gas at a pressure favor- 

 able to stratification (with a feeble spark) we obtain the heaps of 

 powder as before described, it seems sufficiently evident that 

 the causes at work in the case of a feeble discharge are the 

 same, and that though insufficiently strong to displace the 

 powder they are quite marked enough to form the waves of 

 gaseous matter, which are the strata. 



In order to avoid the trouble of constantly exhausting a tube 

 we had a Geissler tube made containing the purest and finest 





