Perhaps the strangest of these is getting a bar of ice out of a red-hot platinum crucible. The object of using platinum is simply to resist the intense heat of the furnace in which the crucible is placed. Pour in sulphurous acid and then fill up with water. The cold raised by vaporising the acid is so intense that the water will freeze into a solid mass. Indeed, the temperature sometimes goes down to more than eighty degrees below freezing. A still more striking experiment is that resulting from the liquefying of nitrous oxide—protoxide of nitrogen, or laughing-gas. This gas needs, as was said, great pressure to liquefy it at an ordinary temperature. At freezing point only a pressure of thirty atmospheres is needed to liquefy it. It then boils if exposed to the air, radiating cold—or, rather, absorbing heat—till it falls to a temperature low enough to freeze mercury. But it still, wonderful to say, retains the property which, alone of all the gases, it shares with oxygen—of increasing combustion. A match that is almost extinguished burns up again quite brightly when thrust into a bag of ordinary laughing-gas; while a bit of charcoal, with scarcely a spark left in it, glows to the intensest white heat when brought in contact with this same gas in its liquid form, so that you have the charcoal at, say, two thousand degrees Fahrenheit, and the gas at some one hundred and fifty degrees below zero. Carbonic acid gas is just the opposite of nitrous oxide, in that it quenches fire and destroys life; but, when liquefied, it develops a like intense cold. Liquefy it and collect it under pressure, in strong cast-iron vessels, and then suddenly open a tap and allow the vapor to escape. In expanding, it grows so cold—or, strictly speaking, absorbs, makes latent, so much heat—that it produces a temperature low enough to turn it into fog and then into frozen fog, or snow. This snow can be gathered in iron vessels, and mixed with either it forms the strongest freezing mixture known, turning mercury into something like lead, so that you can beat the frozen metal with wooden mallets and can mould it into medals and such-like.