(1115 ) 
on the source of light we have used. It may be expected, that 
when the ionisationroom contains the same gas which fills the 
discharge tube, this gas will be more influenced by the ionizing 
rays than any other. 
In the following pages a short account will be given of a research 
after the existence of similar selective effects in the ionization of 
gases under the influence of light emitted by Geissler tubes. For 
particulars we refer to a dissertation which is to appear within a 
few weeks. 
Three methods may be followed in order to find out whether 
ions are formed in a ges by radiation. Firstly we can show the 
existence of ions in damp gas by condensing the aqueous vapour on 
them. Secondly we can make the gas pass along a charged conductor 
or through a condensator after it has been struck by the ionizing 
rays; if the gas has got conductivity the conductor or the condensator 
will be discharged. And in the third place, if the gas under 
examination is at rest between the plates of a condensator or in 
the neighbourhood of a charged conductor, then there will be a 
current in the condensator, or the conductor will lose its charge, 
when the radiation has caused ionization. 
In using the former two methods the risk of erroneous results 
by photoelectric action on minute drops of water or floating dust 
particles is greater than in using the third; therefore the latter 
method was chosen for the research described here. Moreover it 
makes it possible for us to measure the ionization at different pressures 
in the gas under examination. 
The apparatus used for the experiments was made of glass and 
consisted of three separate partitions, viz. the discharge tube, the 
absorptionroom and the ionizationroom. Composed in this way it 
gave an opportunity for examining the absorption of different gases 
for the ionizing rays and so made it possible for us to study an 
eventual connection between ionization and absorption, besides to find 
out the possible existence of selective effects in the absorption also. 
The ionizationroom, or condensatorroom, forms a separate part of 
the apparatus. lt is closed on one end by a ground-in basin-shaped 
piece of glass, the bottom of which is a fluorite plate of 15 m.M. 
diameter and 3 m.M. thickness. On the other side a glass plate is 
cemented against the ground edge. In this plate are two holes, shut 
by amber plugs cemented into them, which are at the same 
