522 On Water and Ah. [August, 
containing the gas. You see the mercury rising higher and 
higher as the pressure is increased, and it is now within a 
couple of inches of the top of the tube, at g, the whole of 
the gas contained in the lower portion of the tube being 
Fig. 42. 
compressed into the small space, g P. Observe the top of 
the column of mercury, and you can see collected there a 
quantity of liquid, clear and colourless ; that is liquid car- 
bonic acid gas. By this compression we have actually 
changed the gas into a liquid. You see that it is atftually 
boiling before you, even at this temperature. If I lower the 
pressure it will boil still more violently ; the liquid will dis- 
appear, and again become a gas. If I increase the pressure 
the liquid gas is again formed. Now what I want you to 
observe is the effedft of the sudden expansion of this atmo- 
sphere above the column of liquid carbonic acid. If you 
take a champagne bottle, say half empty, allowing the car- 
bonic acid to fill the space above the champagne, and sud- 
denly open it, you hear an explosion, and you find the space 
which was formerly perfectly clear above the liquid filled 
with cloud. That is due to the chilling by expansion which 
I illustrated a lecfture or two ago. Now I will try to produce 
