
of Demonstrating the Lonisation of Air. 683 
account of the much larger size of the new apparatus, the 
mechanism by which the sudden expansion is produced was 
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constructed, however, of brass, not, as in the older experi- 
ments, of glass. The cloud chamber A, in which the drops 
formed by expansion are viewed, is a glass cylinder 18°5 cms. 
in internal diameter and 5-9 ems. high. Its roof consists of 
a thick brass disk cemented to it by means of sealing-wax. 
The cylinder rests on an indiarubber ring lying on an 
annular brass plate F, which forms a flange at the top of the 
expansion cylinder B. The glass cylinder is squeezed down 
on the indiarubber by means of an upper annular brass 
plate R resting on the roof of the cloud chamber, from which 
it is separated by a second indiarubber ring ; the upper 
and lower annular plates are connected by six bolts, by means 
of which the necessary pressure can be applied. The external 
diameter of the annular plates is 26 cms. ; about one cm. from 
the edge of the lower one on its upper surface a thin ring L of 
brass 1°2 cm. high is soldered. This serves to contain water, 
all risk of air leaking in below the edge of the glass cylinder 
being thus removed. Through three symmetrically placed 
tubes penetrating the lower plate of the cloud chamber are 
