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Several explanations are given of the temperature of water. Sometimes it is hot, sometimes it boils so fiercely that it cannot be used until it has given off its steam in the open, or is tempered by mixing cold water with it. Empedocles is of opinion that as there are fires concealed in many places beneath the earth, water is heated when they happen to lie beneath the ground through which it has to flow. Let me use an illustration. We are in the habit of constructing serpentines,58 and cylinders, and vessels of several other designs in which thin copper pipes are laid in descending spiral coils. The object is to make the water meet the same fire over and over again, and flow through a space sufficient for heating it up; so, entering as cold it comes out hot. Empedocles supposes something of the same kind to take place underground. People who have their baths heated without fire may well believe that he is right. In this case air from the heated furnace is introduced. The air glides along the passages, warming up the walls and vessels of the bathroom just as if fire had been directly applied. In short, all the cold water in these instances is changed into hot by merely passing through a heated medium; and inasmuch as it is conveyed in an enclosure there is no evaporation to impart a flavour to it.59 Others, again, suppose that the water contracts heat by issuing from or passing through ground charged with sulphur; the heat is imparted by the properties of the material, to which also smell and taste bear witness. All substances, I may say in general terms, tend to reproduce the qualities of the medium by which they have been warmed. If you are surprised at sulphur warming water, you have only to pour water over quicklime; it will at once evolve heat.