In some cases, however, they are filled with fused acetate of soda; this salt is solid when cold, but when the can containing it is heated by immersion in hot water it liquefies, and in the process absorbs heat which is given out again on the change of state back to solid.
Nitrogen peroxide is also prepared by heating lead nitrate and passing the products of decomposition through a tube surrounded by a freezing mixture, when the gas liquefies.
The metal as obtained in this process is lustrous and takes a polish, does not melt in the oxyhydrogen flame, but liquefies in the electric arc, and is not affected by air at ordinary temperatures.
On these occasions the substance contained in the phial liquefies, and the Neapolitans see in this phenomenon a supernatural manifestation.
On the other hand, if a gas be sufficiently cooled and compressed, it liquefies; this transition is treated theoretically in the article Condensation Of Gases, and experimentally in the article Liquid Gases.