1895.] by Electric Discharge. 15 



fair to presume that the luminosity and the increase in volume 

 are strictly simultaneous. If the charge upon the condenser 

 plates be so increased that two or three luminous induction- 

 discharges occur before the discharge of the condenser by the 

 passing of the spark at the Wimshurst, two or three movements 

 of the gauge are correspondingly observable. 



The minimum strength of charge capable of producing these 

 effects varies slightly on different days according to temperature 

 and pressure, but roughly speaking, at ordinary temperatures and 

 pressures the distance between the Wimshurst terminals must 

 be equal to the distance between the two tubes. That is to say, 

 in my experience it has never required to be greater, nor have 

 I been able to obtain any effect if the space between the terminals 

 were more than 23 mm. less than the space between the tubes. 

 Thus with the first tubes used, distant from each other about 

 4 mm., a spark 3'4 mm. was the smallest which produced any 

 effect, whilst with the larger apparatus subsequently used having 

 an air space = 9 mm. the minimum effective spark varied from 65 

 to 9 mm. but was generally about 8 mm. 



These observations were repeated with carbonic acid and coal 

 gas but for a long time with no very satisfactory results, both 

 these gases appearing to give very much the same effects as air — 

 except of course that the colour of the induction- discharge was 

 different. 



Meissner, however, is so very positive in his assertion that H 

 gives distinctly smaller, and C0 2 distinctly greater, effects than 

 air that I could not but think some essential point must have 

 been overlooked. The cause for my failure in obtaining these 

 differences at length became apparent. Meissner seems always to 

 have first filled his vessels with the gas to be observed and then 

 to have charged and discharged them. In my arrangements how- 

 ever it was found more convenient to do the charging and dis- 

 charging by mechanical means, and the Wimshurst which was 

 worked by a small motor was turned on at the beginning of the 

 observations and left to run undisturbed to the end ; thus the gas, 

 which was always admitted gradually into the vessel, was sub- 

 jected to any chemical effects of the discharge before being put 

 into communication with the gauge, the top of which had of 

 course to be kept closed till the vessel was full of gas. Under 

 these circumstances the three gases used, dried air, carbon dioxide 

 and coal gas seem to behave practically identically. But if now 

 the Wimshurst were disconnected whilst the tube was being 

 exhausted and refilled, it was found that, on connecting it up 

 again and discharging frequently as before, air underwent a con- 

 siderable diminution of volume, and whilst this diminution was in 

 progress sudden changes of potential would produce marked 



