Cream from a famous dairy is sent to particular patrons in Paris, France, and it is known that in one instance, at least, a bottle of cream, having failed to reach the person to whom it was consigned, made the return transatlantic voyage and was received in New York three weeks after its first departure, perfectly sweet and good.  Throughout the entire journey it was kept at freezing temperature by artificial means.  These are but two striking examples of wonders that are performed every day.

[Illustration:  THE TYPE MOULDS Moulds for 225 different characters are contained in this frame.]

Cold, of course, is but the absence of heat, and so refrigerating machinery is designed to extract the heat from whatever substance it is desired to cool.  The refrigerating agent used to extract the heat from the cold chamber must in turn have the heat extracted from it, and so the process must be continuous.

Water, when it boils and turns into steam or vapour, is heated by or extracts heat from the fire, but water vapourises at a high temperature and so cannot be used to produce cold.  Other fluids are much more volatile and evaporate much more easily.  Alcohol when spilt on the hand dries almost instantly and leaves a feeling of cold—­the warmth of the hand boils the alcohol and turns it into vapour, and in doing so extracts the heat from the skin, making it cold; now, if the evaporated alcohol could be caught and compressed into its liquid form again you would have a refrigerating machine.

Alcohol is expensive and inflammable, and many other volatile substances have been discarded for the one or the other reason.  Of all the fluids that have been tried, ammonia has been found to work most satisfactorily; it evaporates at a low temperature, is non-inflammable, and is comparatively cheap.

The hold of the supply-ship mentioned at the head of this chapter was a vast refrigerator, but no ice was used except that produced mechanically by the power in the ship.  To produce the cold in the hold of the ship it was necessary to extract the heat in it; to accomplish this, coils ran round the space filled with cold brine, which, as it grew warm, drew the heat from the air.  The brine in turn circulated through a tank containing pipes filled with ammonia vapour which extracted the heat from it; the brine then was ready to circulate through the coils in the hold again and extract more heat.  The heat-extracting or cooling power of the ammonia is exerted continually by the process described below.  Ammonia requires heat to expand and turn into vapour, and this heat it extracts from the substance surrounding it.  In this marine refrigerating machine the ammonia got the heat from the brine in the tank, then it was drawn by a pump from the pipes in the tank, compressed by a power compressor, and forced into a second coil.  The second coil is called a condenser because the vapour was there condensed into a fluid again.