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generally predominates in them. So, too, are the liquids out of

which natron and salt are formed; and stones are also formed from some

mixtures of this kind. Hence, if the whey has not been separated, it

burns away if you boil it over a fire. But the earthy element in

milk can also be coagulated by the help of fig-juice, if you boil it

in a certain way as doctors do when they treat it with fig-juice,

and this is how the whey and the cheese are commonly separated.

Whey, once separated, does not thicken, as the milk did, but boils

away like water. Sometimes, however, there is little or no cheese in

milk, and such milk is not nutritive and is more like water. The

case of blood is similar: cold dries and so solidifies it. Those kinds

of blood that do not solidify, like that of the stag, belong rather to

water and are very cold. Hence they contain no fibres: for the

fibres are of earth and solid, and blood from which they have been

removed does not solidify. This is because it cannot dry; for what

remains is water, just as what remains of milk when cheese has been

removed is water. The fact that diseased blood will not solidify is

evidence of the same thing, for such blood is of the nature of serum

and that is phlegm and water, the nature of the animal having failed

to get the better of it and digest it.

Some of these bodies are soluble, e.g. natron, some insoluble,

e.g. pottery: of the latter, some, like horn, can be softened by heat,

others, like pottery and stone, cannot. The reason is that opposite

causes have opposite effects: consequently, if solidification is due

to two causes, the cold and the dry, solution must be due to the hot

and the moist, that is, to fire and to water (these being

opposites): water dissolving what was solidified by fire alone, fire

what was solidified by cold alone. Consequently, if any things

happen to be solidified by the action of both, these are least apt

to be soluble. Such a case we find where things have been heated and

are then solidified by cold. When the heat in leaving them has

caused most of the moisture to evaporate, the cold so compacts these

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