684 Mr. C. T. R. Wilson on the Condensation Method 
sealed three insulated brass rods supporting a horizontal brass 
disk D, 15°3 cms. in diameter. Between this disk and the 
roof of the cloud chamber, 4°7 cms. above it, any desired 
difference of potential could be maintained by means of a 
battery of storage-cells. Both this disk and that forming the 
roof were covered on the surfaces facing one another with 
wet filter-paper. In addition to the three tubes through which 
pass the supports of the brass disk, the floor of the vessel is 
pierced by a fourth smaller tube T, by means of which air 
can be removed from or admitted into the apparatus. 
Below the cloud chamber and supporting it is a vertical 
brass expansion cylinder B, 10 cms. in internal diameter and 
30 cms. long. Sliding freely in this and serving as a piston 
is a thin-walled brass cylinder open below and with a hemi- 
spherical top, the length of the cylindrical part being 
18°75 cms., the thickness of the walls being less than one 
millimetre. The expansion cylinder is bolted by means of a 
flange at its lower end against a thick brass disk, an india- 
rubber ring, of which the internal diameter is considerably 
less and the external diameter greater than that of the cylinder, 
being inserted between them. Rising up from the centre of 
the disk is a brass tube 18 cms. long and 1°3 cms. in internal 
diameter. ~The cylinder is filled with water to within a few 
cms. from the top of this tube. By means of the mechanism 
to be presently described, the central tube can be put into 
sudden communication with a vacuum chamber V, thus 
causing the piston to fly sharply down against the indiarubber 
at the bottom of the cylinder, and to remain pressed tightly 
against this so that no air or water can escape. It is in this 
way that the sudden expansion is produced: on putting the 
central tube in communication with the atmosphere instead 
of the vacuum chamber, the piston rises to its original 
position. 
The thick brass disk to which the expansion cylinder is 
attached rests upon an iron tripod (not shown in the figure) 
to the top of which it is firmly fixed by three screws. The 
feet of the tripod are screwed down to a board. The tubes 
for making connexion with the vacuum chamber are shown 
below the expansion cylinder. For convenience the con- 
nexions are made with screw-joints, indiarubber washers being 
inserted to prevent leakage. The vacuum chamber was a 
brass cylinder 22 cms. long and 14 cms. in diameter, with 
rounded ends ; it was maintained at low pressure by a water- 
jet pump. A gauge was connected to avoid the risk of making 
an expansion while the vacuum was not sufficiently good. 
The construction of the mechanism for making sudden 
