When the air has been sufficiently cooled it is allowed to pass into a small chamber, expanding as it goes, and from the small into a larger chamber, where the cold of expansion becomes so acute that the air-molecules collapse into liquid, which collects in a special receptacle. Arrangements are made whereby any vapour rising from the liquid passes through a space outside the expansion chambers, so that it helps to cool the incoming air and is not wasted. The liquid-air tank is inside a great wooden case, carefully protected from the heat of the atmosphere by non-conducting substances. A tap being turned, a rush of vapour shoots out, soon followed by a clear, bluish liquid, which is the air we breathe in a fresh guise. A quantity of it is collected in a saucepan. It simmers at first, and presently boils like water on a fire. The air-heat is _by comparison_ so great that the liquid cannot resist it, and strives to regain its former condition. You may dip your finger into the saucepan--if you withdraw it again quickly--without hurt. The cushion of air that your finger takes in with it protects you against harm--for a moment. But if you held it in the liquid for a couple of seconds you would be minus a digit. Pour a little over your coat sleeve. It flows harmlessly to the ground, where it suddenly expands into a cloud of chilly vapour. Put some in a test tube and cork it up. The cork soon flies out with a report--the pressure of the boiling air drives it. Now watch the boiling process. The nitrogen being more volatile--as it boils at a lower temperature than oxygen--passes off first, leaving the pure, blue oxygen. The temperature of this liquid is over 312 degrees below zero (as far below the temperature of the air we breathe as the temperature of molten lead is above it!). A tumbler of liquid oxygen dipped into water is soon covered with a coating of ice, which can be detached from the tumbler and itself used as a cup to hold the liquid. If a bit of steel wire be now twisted round a lighted match and the whole dipped into the cup, the steel flares fiercely and fuses into small pellets; which means that an operation requiring 3000 degrees Fahrenheit has been accomplished in a liquid 300 degrees below zero!