Avicenna says:
“These material bodies are divided into four species: into stones, liquifying agents, sulphurs, and salts. Of these, some have a rarefied substance and a weak compound, some others are strong substances, some are easily bent and some are not. And of those that are weak substances, some are salts that are liquefied easily by moisture, such as calcium, alum and sal armonicaum (probably ammonium chloride). And some are oily and are not easily liquefied by moisture alone, such as sulphur and auripigmentum (probably arsenic trisulphide). …And the matter of things that can be liquefied are strong compounds, mixed from watery substances and earthy substances, that cannot be separated from one another, such as watery substances that are frozen with cold after the application of heat while covered, an example of which is alum. Alum does not freeze on account of its oiliness and can be bent. However among mineral substances, rocks though materially on water, are not solidified by water alone, rather also by a dryness that alters their watery nature making it earthy, and their human is not too, and thus cannot be bent.”
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