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THE EFFECT OF THE ELECTRIC DISCHARGE ON 
WATER VAPOUR. 
By E. Jacot, B.A. 
(Plates II., III.) 
(Communicated by W. H. Logeman, M.A.) 
(Read July 20, 1910.) 
Introductory . — This research was initially undertaken with a view to 
verifying the results set forth by M. A. Henry in the Journal de Physiqite 
(tome viii., Jan., 1909, pp. 33-38), namely, that a vacuum tube containing 
only water vapour will show Cooke's effects at considerably higher 
pressures than if it contained air ; i.e., that the upper limit of pressure 
suitable for the existence of cathode rays, and consequently of Rontgen 
rays, is much higher in the case of water vapour than in the case 
of air. 
At an early stage of the work, however, unexpected effects occurred, 
and the investigation of these rendered the original subject of research 
altogether secondary. 
Method of Procedure. — The method of procedure and the experi- 
mental arrangements were essentially different to those of M. Henry. 
In principle, the method first intended to be employed was the following : 
An ordinary X-ray bulb, fitted with suitable means for admitting various 
quantities of air or water vapour, was connected to a mercury pump. 
Air pressures were read by a McLeod gauge, and pressures of water 
vapour by a manometer of the ordinary kind. Exhaustion was carried on 
until the bulb emitted good X-rays, and the quantity of air or water 
vapour necessary to cause a cessation of the emission of rays was 
then noted. 
Methods of admitting Water Vapour. — The first method of introducing 
water vapour into the bulb was through a fine capillary of such small bore 
that air could be admitted at the rate of twenty to thirty thousandths of 
