450 
ME. T. H. LABY ON THE SUPERSATURATION AND 
at the middle to D and to earth. The potential of the top of the expansion chamber 
and the lower diaphragm C by connecting them to these contacts could be varied 
between +1 and —1 volt (in steps of 0 - 02 volt) with respect to the middle 
diaphragm D. In a number of experiments, the top in one expansion was at — 1 volt, 
and the bottom at +1 ; in the next expansion the potentials of these plates were 
reversed. 
When the Rontgen rays from the bulb R pass through the slit, they ionise above 
and below the diaphragm a thin layer of the mixture of air and the vapour present. 
The secondary rays produced by the primary inside the expansion chamber would 
produce a few ions throughout all the interior. With the diaphragm earthed, the 
top negatively and the bottom positively charged, the ions stream as indicated in 
fig. 4. 
/ 
It then an expansion be made of the dust-free air and vapour (within one second) 
alter cutting off the Bontgen rays, positive ions will be present in the top partition, 
and negative in the bottom, which will act as condensation nuclei, If is to be 
