276 
MR. C. T. R. WILSON ON CONDENSATION OF WATER VAPOUR 
Ave please. Then, on pushing up the plug G, we suddenly reduce the pressure in the 
small space C to that of the atmosphere, and the piston flies back to its original 
position, producing a sudden expansion of the gas in A. 
After each expansion a small quantity of water is driven in through T^. This 
serves to keep the walls of A covered with a film of water and to lubricate the 
piston. The greater part of it runs down and collects above the latter. When the 
Fig. 2. 
pl,ston is driven up again so that it is suspended freely in the tube, It gradually 
floats up through this water till it comes to rest Avith the water scarcely reaching 
above the straight part of its sides. In this way the water which lubricates the 
piston is continually being renewed, and the gradual contamination of the gas Avhich 
would otherwise result bv solution and difliision of the air from below through the 
water is almost indefinitely retarded. 
