GASES BY THE ACTION OF RONTGEN BAYS AND OTHER AGENTS. 
405 
2, Expansion Apparatus. 
The apparatus that I have most frequently used for bringing about the sudden 
expansion required in these experiments is represented in fig. 1. 
A is the cloud chamber, where any drops which may be formed on expansion are 
made visible by the light from a luminous gas flame, brought by means of a lens to 
Fig. 1. 
a locus within it. As the form of the cloud chamber had very frequently to be 
changed, it was convenient to arrange that it could readily be detached from the rest 
of the apparatus. An air-tight joint was made by means of the indiarubber stopper 
and mercury-cup arrangement shown in the diagram. The form of the cloud chamber 
there sho’svn is the simplest that was used. 
The cylindrical glass tube B (internal diameter = 2'7 centim.) is closed at its lower 
end by an indiarubber stopper, through the centre of which passes a glass tube, C, 
about 1 centim. in diameter, with a wider portion at its upper end serving as a guide 
