1895.] by Electric Discharge. 13 



tube when the latter was being exhausted and refilled with what- 

 ever gas might be under observation ; for this purpose a short 

 tube projected between the tap and the bend which communicated 

 with a pump and with the source of gas employed. In this case 

 the inner and outer tube, instead of being cemented together with 

 wax, were provided with a rather loose ring of india-rubber be- 

 tween which and the glass melted paraffin was poured and allowed 

 to harden so that the joint was air-tight. For about 1*5 cm. 

 below the india-rubber the space between the two tubes was 

 completely filled with paraffin. Instead of lining the inside tube 

 with tinfoil it was filled with mercury ; the outer tube as before 

 being coated with tinfoil in which spaces were cut to enable the 

 observer to note what went on inside the tube. This outer tinfoil 

 which extended from just below the lower end of the paraffin 

 stopper to within 4 cm. of the bend was 28 cm. long and had 

 an area of 2903 sq. cm. — not allowing for the small spaces cut out 

 — the effective mercury surface of the inner tube was 87 96 sq. cm. 

 The two surfaces were distant from one another 11 5 cm. and the 

 air space between the two tubes had a width of '9 cm. The two 

 latter measurements may not be perfectly correct as of course 

 it is the smallest distance between the two tubes that is required 

 and, though every care was taken, the two tubes may not be 

 perfectly concentric; there may be an error of 1 or even of 1*5 mm. 

 The condenser thus formed was mounted on a square of glass by 

 means of a thick band of paraffin. The whole was then enclosed 

 in a light-tight case so arranged that the lower part of the appa- 

 ratus forming the gauge projected from an opening beneath, and 

 provided with an opening in front fitted with a small blackened 

 tube through which the interior could be observed. A mm. scale 

 was affixed to the gauge tube. The mercury in the inner tube 

 was connected to the inner coating of one Leyden jar, the tinfoil 

 on the outer tube to another jar, the outsides of the two jars being 

 connected to earth. The maximum charge upon the two tubes 

 could be regulated by means of the distance between the ter- 

 minals of the Wimshurst by which the apparatus was charged. 



With these arrangements it was easily seen that any move- 

 ment of the acid in the gauge of not less than '25 mm. was 

 accompanied by a luminous discharge through the space between 

 the two tubes, but it appeared at first as though smaller move- 

 ments than this were unaccompanied by any luminosity. Greater 

 care however proved that this was not the case, but that every 

 movement of the acid is accompanied by a corresponding lumi- 

 nosity between the tubes. It requires great care and some 

 practice however to see this when at its faintest ; it is much 

 easier to perceive the very slightest shake of the acid than 

 the almost imperceptible luminous discharge which however un- 



