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Liquefaction is, first, condensation into water; second, the melting

of a solidified body. The first, condensation, is due to the cooling

of vapour: what melting is will appear from the account of

solidification.

Whatever solidifies is either water or a mixture of earth and water,

and the agent is either dry heat or cold. Hence those of the bodies

solidified by heat or cold which are soluble at all are dissolved by

their opposites. Bodies solidified by the dry-hot are dissolved by

water, which is the moist-cold, while bodies solidified by cold are

dissolved by fire, which is hot. Some things seem to be solidified

by water, e.g. boiled honey, but really it is not the water but the

cold in the water which effects the solidification. Aqueous bodies are

not solidified by fire: for it is fire that dissolves them, and the

same cause in the same relation cannot have opposite effects upon

the same thing. Again, water solidifies owing to the departure of

heat; so it will clearly be dissolved by the entry into it of heat:

cold, therefore, must be the agent in solidifying it.

Hence aqueous bodies do not thicken when they solidify; for

thickening occurs when the moisture goes off and the dry matter

comes together, but water is the only liquid that does not thicken.

Those bodies that are made up of both earth and water are solidified

both by fire and by cold and in either case are thickened. The

operation of the two is in a way the same and in a way different. Heat

acts by drawing off the moisture, and as the moisture goes off in

vapour the dry matter thickens and collects. Cold acts by driving

out the heat, which is accompanied by the moisture as this goes off in

vapour with it. Bodies that are soft but not liquid do not thicken but

solidify when the moisture leaves them, e.g. potter's clay in

process of baking: but those mixed bodies that are liquid thicken

besides solidifying, like milk. Those bodies which have first been

thickened or hardened by cold often begin by becoming moist: thus

potter's clay at first in the process of baking steams and grows

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